How to retrieve a client certificate on the server side sent with a request

See more:

C#

I have a client application which make a request to server page. Code below:

HttpWebRequest req = (HttpWebRequest)WebRequest.Create("https://testpage.aspx");
req.Method = "POST";
req.ClientCertificates.Add(clientcert);
req.ContentLength = 0;

testpage.aspx is a page on the web server and needs to get the certificate sent with the request (i.e. clientcert) and validate it against another one on the active directory.

I tried using Request.ClientCertificate but it is empty.

Please help! Thanks!