Adding HttpRequest ClientCertificate

Hi all;
I am developing a process for a web site that utlizes client certificates for user validation.  On the test page in development, using the built-in Visual Studio development server, I would like to access add a client certificate to the Request object that could then be read in my process using the Cert_Subject Server variable.  I saw a thread showing how one was able to add a certificate to a HttpWebRequest object using the ClientCertificates.Add(cer) method, after creating an X509Certificate object from a ".cer" file.  However, it is not possible to cast the HttpRequest object to HttpWebRequest in order to carry this out.  The HttpRequest Object's ClientCertificate.Add method only adds another Name value pair to the certificate. 
Does anyone know of a way I can accomplish this running the web application in debug on VS's built-development web server?
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