How can I add a SOAP authentication header with HTTPRequestMessage?

Question

Here is what the header is supposed to look like

<soap:Header>
   <AuthenticationHeader>
     <UserName>string</UserName>
     <Password>string</Password>
   </AuthenticationHeader>
 </soap:Header>

Here is what I've tried:

string username = "TheUserName";
string password = "ThePassword";

HttpRequestMessage requestMessage = new HttpRequestMessage(method, uri);
requestMessage.Headers.Add("UserName", username);
requestMessage.Headers.Add("Password", password);

Maybe I have to somehow set the authorization header?

requestMessage.Headers.Authorization = ??

I feel like somehow I have to "build" that AuthenticationHeader element but I'm not sure how to do that. Any suggestions?

Edit: Full SOAP Envelope

?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Header>
    <AuthenticationHeader xmlns="http://www.test.com/testing/Security">
      <UserName>string</UserName>
      <Password>string</Password>
    </AuthenticationHeader>
  </soap:Header>
  <soap:Body>
    <GetMeSomething xmlns="http://www.test.com/testing/WorkFileCatalog">
      <Param1>string</Param1>
      <Param2>string</Param2>
      <XMLRetMess>string</XMLRetMess>
    </GetMeSomething>
  </soap:Body>
</soap:Envelope>

Answer 1

Given the provided OP, the following Unit Test was done as a proof of concept of how you can populate the header message header and create a request.

[TestClass]
public class SOAP_UnitTests {
    private HttpMethod method;
    private string uri;
    private string action;

    [TestMethod]
    public void _Add_SOAP_Auth_Header_Details_With_HttpRequestMessage() {
        string username = "TheUserName";
        string password = "ThePassword";

        var xml = ConstructSoapEnvelope();
        var doc = XDocument.Parse(xml);
        var authHeader = doc.Descendants("{http://www.test.com/testing/Security}AuthenticationHeader").FirstOrDefault();
        if (authHeader != null) {
            authHeader.Element(authHeader.GetDefaultNamespace() + "UserName").Value = username;
            authHeader.Element(authHeader.GetDefaultNamespace() + "Password").Value = password;
        }
        string envelope = doc.ToString();

        var request = CreateRequest(method, uri, action, doc);
        request.Content = new StringContent(envelope, Encoding.UTF8, "text/xml");

        //request is now ready to be sent via HttpClient
        //client.SendAsync(request);
    }

    private static HttpRequestMessage CreateRequest(HttpMethod method, string url, string action, XDocument soapEnvelopeXml) {
        var request = new HttpRequestMessage(method: method, requestUri: url);
        request.Headers.Add("SOAPAction", action);
        request.Headers.Add("ContentType", "text/xml;charset=\"utf-8\"");
        request.Headers.Add("Accept", "text/xml");
        request.Content = new StringContent(soapEnvelopeXml.ToString(), Encoding.UTF8, "text/xml"); ;
        return request;
    }

    private string ConstructSoapEnvelope() {
        var message = @"<?xml version='1.0' encoding='utf-8'?>
<soap:Envelope xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance' xmlns:xsd='http://www.w3.org/2001/XMLSchema' xmlns:soap='http://schemas.xmlsoap.org/soap/envelope/'>
  <soap:Header>
    <AuthenticationHeader xmlns='http://www.test.com/testing/Security'>
      <UserName>string</UserName>
      <Password>string</Password>
    </AuthenticationHeader>
  </soap:Header>
  <soap:Body>
    <GetMeSomething xmlns='http://www.test.com/testing/WorkFileCatalog'>
      <Param1>string</Param1>
      <Param2>string</Param2>
      <XMLRetMess>string</XMLRetMess>
    </GetMeSomething>
  </soap:Body>
</soap:Envelope>
";
        return message;
    }
}