If you use C #, the class HttpWebRequesthas a collection property clientCertificates. In short, before submitting a request, you add your certificate to this collection.
Do you want to make some stream along the lines
#:
(, 100%):
HttpWebRequest req = (HttpWebRequest)WebRequest.Create(requestUrl);
X509Store keystore = new X509Store("My", StoreLocation.CurrentUser);
X509CertificateCollection certs = keystore.Certificates.Find(X509FindType.FindBySubjectName, "Name Of Cert", true);
req.ClientCertificates = certs;