How can I "Emulate" XMLHTTPRequest?

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C#

Javascript

XML

HTTP

Hi so I've recently come around a home page that allows you to copy annotations in a youtube video on to another using some base javascript functionality how ever I'm now trying to expand upon this and create a more advance annotation creator, using C#.

The scripts can be fond on this site: http://stefansundin.com/stuff/youtube/youtube-copy-annotations.html[^] but the may part of the script I'm struggling with is:

var xhr = new XMLHttpRequest();
xhr.withCredentials = true;
xhr.open('POST', 'https://www.youtube.com/annotations_auth/update2', true);
xhr.send(data);

For now I've tried using the WebRequest and HTTPWebRequest but it keep throwing me an 403 error;

string xData = xDoc.InnerXml;
            
            byte[] buffer = System.Text.Encoding.ASCII.GetBytes(xData);

            WebRequest rq = (WebRequest)WebRequest.Create("https://www.youtube.com/annotations_auth/update2");
            rq.Method = "POST";
            
            using (Stream s = rq.GetRequestStream())
            {
                s.Write(buffer, 0, buffer.Length);
            }

            HttpWebResponse httpWebResponse = (HttpWebResponse)rq.GetResponse(); //Runtime error The remote server returned an error: (403) Forbidden. 

I've already triple - if not quadruple - checked the XML content and it should be completely identical with the one I send on the site, where the only exception may be the encoding?

So do anyone know how I can change this JavaScript into something shiny/working in C#?

Thanks a bunch in regard!