403 Forbidden Error Due to Wrong Impersonation Level

I'm using HttpWebRequest and HttpWebResponse to query a remote server. I plan to load the returned xml into an XMLDocument, but the HttpWebRequest.GetResponse() method fails with a 403 "Forbidden" error.

I am able to get a response when I put my domain username and password as plain text into the request's Credentials member, but of course I'd like to avoid having that information in the application.

I thought that code along these lines (found at )
would solve my problem:

using <!--coloro:#408080--><span style="color:#408080 <!--/coloro-->System.Security.Principal;<!--colorc--><!--/colorc-->

<!--coloro:#0000ff--><span style="color:#0000ff <!--/coloro-->if<!--colorc--><!--/colorc--> (User.GetType() == <!--coloro:#0000ff--><span style="color:#0000ff <!--/coloro-->typeof<!--colorc--><!--/colorc-->(<!--coloro:#408080--><span style="color:#408080 <!--/coloro-->WindowsPrincipal<!--colorc--><!--/colorc-->))
{
<!--coloro:#408080--><span style="color:#408080 <!--/coloro-->WindowsIdentity<!--colorc--><!--/colorc--> id = (<!--coloro:#408080--><span style="color:#408080 <!--/coloro-->WindowsIdentity<!--colorc--><!--/colorc-->) User.Identity;
<!--coloro:#408080--><span style="color:#408080 <!--/coloro-->WindowsImpersonationContext<!--colorc--><!--/colorc--> impersonate = id.Impersonate();

<!--coloro:#008000--><span style="color:#008000 <!--/coloro-->//perform tasks under the impersonated user
//*** ***// <!--colorc--><!--/colorc-->
<!--coloro:#008000--><span style="color:#008000 <!--/coloro-->//revert back to local ASPNET account<!--colorc--><!--/colorc-->
impersonate.Undo();
}
<!--coloro:#0000ff--><span style="color:#0000ff <!--/coloro-->else<!--colorc--><!--/colorc-->
{
<!--coloro:#008040--><span style="color:#008040 <!--/coloro-->//user isn’t authenticated<!--colorc--><!--/colorc-->
}
But, I still get the 403 error. I think the impersonation is failing because the WindowsIdentity.ImpersonationLevel property is always "Impersonation." I think this needs to be "Delegation" to reach the remote server (I'm behind a firewall and corporate proxy), but the property is read-only so I can't set it.

My Question: What do I need to configure in order to get the ImpersonationLevel property to be of the TokenImpersonationLevel.Delegation type?

Let me know if this requires clarification.