how to send client certificate in http request?

Question

I want to send client certificate in HttpWebRequest.

this is my code to build http web request.

 private static HttpWebRequest CreateWebRequest(string url, string action)
        {
            string UserName = "*******";
            string Password = "******";

            string auth = "Basic " + Convert.ToBase64String(System.Text.Encoding.Default.GetBytes(UserName + ":" + Password));

            HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(url);
            webRequest.Headers.Add("SOAPAction", action);
            webRequest.Headers.Add("Authorization", auth);
            webRequest.ContentType = "text/xml;charset=\"utf-8\"";
            webRequest.Accept = "text/xml";
            webRequest.Method = "POST";
            return webRequest;
        }

I am new into HttpWebRequest.