How to send data as a SoapMessage and get a reply?

Personally, I use ServiceStack to create both client and server

https://docs.servicestack.net/soap-support

Or SoapHttpClient nuget

https://github.com/pmorelli92/SoapHttpClient

Or my example from way back when

Is it possible that I can convert simple string to SOAP Message and send it?

The answer depends on what framework or libraries do you plan to use?

Here is sample C# Console client and server code (they are in the same sample but this is only for demo purpose, of course) that uses HTTPS.

For the client side, we reuse the SoapHttpClientProtocol class, but for the server side, unfortunately, we cannot reuse anything because classes are completely tied to ASP.NET's (IIS) HttpContext class

For the server side, we use HttpListener, so, depending on your configuration, the server side will probably require admin rights to be able to call HttpListener's Prefixes.Add(url).

The code doesn't uses client certificate, but you can add this where I placed // TODO comments

The code assumes there is a certificate associated with the url and port used. If there's not (use netsh http show sslcert to dump all associated certs), you can use the procedure described here to add one: https://stackoverflow.com/a/11457719/403671

using System;
using System.IO;
using System.Net;
using System.Text;
using System.Threading.Tasks;
using System.Web.Services;
using System.Web.Services.Protocols;
using System.Xml;

namespace SoapTests
{
    class Program
    {
        static void Main(string[] args)
        {
            // code presumes there is an sslcert associated with the url/port below
            var url = "https://127.0.0.1:443/";
            using (var server = new MyServer(url, MyClient.NamespaceUri))
            {
                server.Start(); // requests will occur on other threads
                using (var client = new MyClient())
                {
                    client.Url = url;
                    Console.WriteLine(client.SendTextAsync("hello world").Result);
                }
            }
        }
    }

    [WebServiceBinding(Namespace = NamespaceUri)]
    public class MyClient : SoapHttpClientProtocol
    {
        public const string NamespaceUri = "http://myclient.org/";

        public async Task<string> SendTextAsync(string text)
        {
            // TODO: add client certificates using this.ClientCertificates property
            var result = await InvokeAsync(nameof(SendText), new object[] { text }).ConfigureAwait(false);
            return result?[0]?.ToString();
        }

        // using this method is not recommended, as async is preferred
        // but we need it with this attribute to make underlying implementation happy
        [SoapDocumentMethod]
        public string SendText(string text) => SendTextAsync(text).Result;

        // this is the new Task-based async model (TAP) wrapping the old Async programming model (APM)
        public Task<object[]> InvokeAsync(string methodName, object[] input, object state = null)
        {
            if (methodName == null)
                throw new ArgumentNullException(nameof(methodName));

            return Task<object[]>.Factory.FromAsync(
                beginMethod: (i, c, o) => BeginInvoke(methodName, i, c, o),
                endMethod: EndInvoke,
                arg1: input,
                state: state);
        }
    }

    // server implementation
    public class MyServer : TinySoapServer
    {
        public MyServer(string url, string namespaceUri)
            : base(url)
        {
            if (namespaceUri == null)
                throw new ArgumentNullException(nameof(namespaceUri));

            NamespaceUri = namespaceUri;
        }

        // must be same as client namespace in attribute
        public override string NamespaceUri { get; }

        protected override bool HandleSoapMethod(XmlDocument outputDocument, XmlElement requestMethodElement, XmlElement responseMethodElement)
        {
            switch (requestMethodElement.LocalName)
            {
                case "SendText":
                    // get the input
                    var text = requestMethodElement["text", NamespaceUri]?.InnerText;
                    text += " from server";

                    AddSoapResult(outputDocument, requestMethodElement, responseMethodElement, text);
                    return true;
            }
            return false;
        }
    }

    // simple generic SOAP server
    public abstract class TinySoapServer : IDisposable
    {
        private readonly HttpListener _listener;

        protected TinySoapServer(string url)
        {
            if (url == null)
                throw new ArgumentNullException(nameof(url));

            _listener = new HttpListener();
            _listener.Prefixes.Add(url); // this requires some rights if not used on localhost
        }

        public abstract string NamespaceUri { get; }
        protected abstract bool HandleSoapMethod(XmlDocument outputDocument, XmlElement requestMethodElement, XmlElement responseMethodElement);

        public async void Start()
        {
            _listener.Start();
            do
            {
                var ctx = await _listener.GetContextAsync().ConfigureAwait(false);
                ProcessRequest(ctx);
            }
            while (true);
        }

        protected virtual void ProcessRequest(HttpListenerContext context)
        {
            if (context == null)
                throw new ArgumentNullException(nameof(context));

            // TODO: add a call to context.Request.GetClientCertificate() to validate client cert
            using (var stream = context.Response.OutputStream)
            {
                ProcessSoapRequest(context, stream);
            }
        }

        protected virtual void AddSoapResult(XmlDocument outputDocument, XmlElement requestMethodElement, XmlElement responseMethodElement, string innerText)
        {
            if (outputDocument == null)
                throw new ArgumentNullException(nameof(outputDocument));

            if (requestMethodElement == null)
                throw new ArgumentNullException(nameof(requestMethodElement));

            if (responseMethodElement == null)
                throw new ArgumentNullException(nameof(responseMethodElement));

            var result = outputDocument.CreateElement(requestMethodElement.LocalName + "Result", NamespaceUri);
            responseMethodElement.AppendChild(result);
            result.InnerText = innerText ?? string.Empty;
        }

        protected virtual void ProcessSoapRequest(HttpListenerContext context, Stream outputStream)
        {
            // parse input
            var input = new XmlDocument();
            input.Load(context.Request.InputStream);

            var ns = new XmlNamespaceManager(new NameTable());
            const string soapNsUri = "http://schemas.xmlsoap.org/soap/envelope/";
            ns.AddNamespace("soap", soapNsUri);
            ns.AddNamespace("x", NamespaceUri);

            // prepare output
            var output = new XmlDocument();
            output.LoadXml("<Envelope xmlns='" + soapNsUri + "'><Body/></Envelope>");
            var body = output.SelectSingleNode("//soap:Body", ns);

            // get the method name, select the first node in our custom namespace
            bool handled = false;
            if (input.SelectSingleNode("//x:*", ns) is XmlElement requestElement)
            {
                var responseElement = output.CreateElement(requestElement.LocalName + "Response", NamespaceUri);
                body.AppendChild(responseElement);

                if (HandleSoapMethod(output, requestElement, responseElement))
                {
                    context.Response.ContentType = "application/soap+xml; charset=utf-8";
                    context.Response.StatusCode = (int)HttpStatusCode.OK;
                    var writer = new XmlTextWriter(outputStream, Encoding.UTF8);
                    output.WriteTo(writer);
                    writer.Flush();
                    handled = true;
                }
            }

            if (!handled)
            {
                context.Response.StatusCode = (int)HttpStatusCode.BadRequest;
            }
        }

        public void Stop() => _listener.Stop();
        public virtual void Dispose() => _listener.Close();
    }
}