How do I set multiple headers using PostAsync in C#?

I stopped using the Post/Get *Async methods in favor of the SendAsync(...) method and HttpRequestMessage Send Async is the big brother which allows you the full flexibility you otherwise can't achieve.

using System.Net.Http;

var httpRequestMessage = new HttpRequestMessage();

httpRequestMessage.Method = httpMethod;
httpRequestMessage.RequestUri = new Uri(url);

httpRequestMessage.Headers
                  .UserAgent
                  .Add(new Headers.ProductInfoHeaderValue(
                                              _applicationAssembly.Name, 
                                              _applicationAssembly.Version.ToString()));

HttpContent httpContent = new StringContent(json, Encoding.UTF8, "application/json");
switch (httpMethod.Method)
{
    case "POST":
        httpRequestMessage.Content = httpContent;
        break;
}

var result = await httpClient.SendAsync(httpRequestMessage);
result.EnsureSuccessStatusCode();

You can also use

var client = new HttpClient();
client.DefaultRequestHeaders.TryAddWithoutValidation("headername","headervalue");

If you want to just set the headers on the HttpClient class just once. Here is the MSDN docs on DefaultRequestHeaders.TryAddWithoutValidation

You can access the Headers property through the StringContent:

var content = new StringContent(Request, Encoding.UTF8, header);
content.Headers.Add(...);

Then pass the StringContent to the PostAsync call:

response = client.PostAsync(Url, content).Result;