A simple reverse proxy using ASP.NET, C# with authentication

I am attempting to forward custom parameters to a RESTful API server and return the proxied response to the client-facing server. I don't want the client to have access to or be able to read the API HTTP request/response interactions, so I decided to perform this action using a reverse proxy. I have no problem forwarding the request and returning a response. The problem lies in the authentication. The client-facing server always wants to redirect to the login page because it doesn't believe the client is authenticated. I have tried using HTTPS and HTTP with similar results.

I have been researching this problem for quite some time and found quite a variety of answers, none of which seem to quite encompass my specific use case. I am following this example, which is the closest to what I specifically need. However, the credentials portion the author commented out (//request.Credentials = CredentialCache.DefaultCredentials;) doesn't seem to cover the authentication portion I am attempting to implement. Please help me understand this problem and solution.

Here is the code I am using from the controller:

public ActionResult ProxyEndpoint(string custom_string, string another_custom_string)
{
    //Bunch of code here to grab the remoteUrl from AppConfig and do stuff to the parameters and store them in queryString, unnecessary to show here.

    //Here's the important bits:
    remoteUrl = remoteUrl + "?" + queryString; // create my remoteUrl
    HttpWebRequest request = (HttpWebRequest)WebRequest.Create(remoteUrl);
    request.Credentials = CredentialCache.DefaultCredentials;
    // Also tried this to no avail:
    request.Credentials = CredentialCache.DefaultNetworkCredentials;
    return ProxyActionResult(request.GetResponse());
}

Here is the ProxyActionResult class:

public class ProxyActionResult : ActionResult
{
    WebResponse _response;

    public ProxyActionResult(WebResponse response)
    {
        _response = response;
    }

    public override void ExecuteResult(ControllerContext controllerContext)
    {
        HttpContextBase httpContext = controllerContext.HttpContext;
        WebResponse response = _response;

        // Read the byte stream from the response:
        Stream responseStream = response.GetResponseStream();

        // Pulled this next piece from http://www.codeproject.com/Articles/7135/Simple-HTTP-Reverse-Proxy-with-ASP-NET-and-IIS
        // Seemed to fit our use case.
        if ((response.ContentType.ToLower().IndexOf("html") >= 0) || (response.ContentType.ToLower().IndexOf("javascript") >= 0))// || (response.ContentType.ToLower().IndexOf("image") >= 0))
        {
            //If the response is HTML Content, parse it like HTML:
            StreamReader readStream = new StreamReader(responseStream, Encoding.Default);

            String content;

            content = ParseHtmlResponse(readStream.ReadToEnd(), httpContext.Request.ApplicationPath);

            //Write the updated HTML to the client(and then close the response):
            httpContext.Response.Write(content);
            httpContext.Response.ContentType = response.ContentType;

            response.Close();
            httpContext.Response.End();
        }
        else
        {
            // If the response is not HTML Content, write the stream directly to the client:
            var buffer = new byte[1024];
            int bytes = 0;
            while ((bytes = responseStream.Read(buffer, 0, 1024)) > 0)
            {
                httpContext.Response.OutputStream.Write(buffer, 0, bytes);
            }
            // from http://www.dotnetperls.com/response-binarywrite
            httpContext.Response.ContentType = response.ContentType; // Set the appropriate content type of the response stream.
            // and close the stream:
            response.Close();
            httpContext.Response.End();
        }
        //throw new NotImplementedException();
    }
    // Debating whether we need this:
    public string ParseHtmlResponse(string html, string appPath)
    {
        html = html.Replace("\"/", "\"" + appPath + "/");
        html = html.Replace("'/", "'" + appPath + "/");
        html = html.Replace("=/", "=" + appPath + "/");
        return html;
    }