How do you add a swagger comment to the "Request and Response Model"?

I'm using .net core 3.0, so in addition to @Helder's response, I had to do below two more steps to make XML comments visible.

manually edit the project file.

<PropertyGroup>
  <GenerateDocumentationFile>true</GenerateDocumentationFile>
  <NoWarn>$(NoWarn);1591</NoWarn>
</PropertyGroup>

add below to startup.cs service configuration method.

services.AddSwaggerGen(c =>
            {
                c.SwaggerDoc("v1", new OpenApiInfo
                {
                    Title = "My Good API",
                    Version = "v1",
                    Description = "Doesn't hurt to add some description."
                });

                // Set the comments path for the Swagger JSON and UI.
                var xmlFile = $"{Assembly.GetExecutingAssembly().GetName().Name}.xml";
                var xmlPath = Path.Combine(AppContext.BaseDirectory, xmlFile);
                c.IncludeXmlComments(xmlPath);
            });

From the microsoft docs for Swagger

XML comments can be enabled with the following approaches:

• Manually add the highlighted lines to the .csproj file:

Enabling XML comments provides debug information for undocumented public types and members. Undocumented types and members are indicated by the warning message. For example, the following message indicates a violation of warning code 1591:

Yes just like Dimitar said, you can add comments to the responses with SwaggerResponse, the request is a bit different, just like you added xml comments to your action you should add to the parameters, here is an example:

using Swagger.Net.Annotations;
using System;
using System.Collections.Generic;
using System.Net;
using System.Web.Http;
using System.Web.Http.Results;

namespace Swagger_Test.Controllers
{
    public class IHttpActionResultController : ApiController
    {

        [SwaggerResponse(HttpStatusCode.OK, "List of customers", typeof(IEnumerable<int>))]
        [SwaggerResponse(HttpStatusCode.NotFound, Type = typeof(NotFoundResult))]
        public IHttpActionResult Post(MyData data)
        {
            throw new NotImplementedException();
        }
    }

    /// <summary>My super duper data</summary>
    public class MyData
    {
        /// <summary>The unique identifier</summary>
        public int id { get; set; }

        /// <summary>Everyone needs a name</summary>
        public string name { get; set; }
    }
}

And in swagger that will look like: