How can I add a SOAP authentication header with HTTPRequestMessage?

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>