ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3;

WebRequest request = WebRequest.Create(url);

request.Credentials = CredentialCache.DefaultCredentials;
request.Method = "POST";

//(request as HttpWebRequest).ClientCertificates.Add(X509Certificate2.CreateFromCertFile(ConfigurationManager.AppSettings["ClientSertificatePath"]));

X509Certificate2 ser = new X509Certificate2(ConfigurationManager.AppSettings["ClientSertificatePath"], "******");

(request as HttpWebRequest).ClientCertificates.Add(ser);

//(request as HttpWebRequest).CookieContainer = new CookieContainer();
//(request as HttpWebRequest).Accept = "*/*";
//(request as HttpWebRequest).AllowAutoRedirect = false;

//(request as HttpWebRequest).UserAgent = "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.2; SV1; .NET CLR 1.1.4322)";
(request as HttpWebRequest).UserAgent = "Network Processing";
//(request as HttpWebRequest).KeepAlive = false;
//(request as HttpWebRequest).ProtocolVersion = HttpVersion.Version11;
//(request as HttpWebRequest).PreAuthenticate = true;

string postData = xml;

byte[] byteArray = Encoding.UTF8.GetBytes(postData);

request.ContentType = "application/x-www-form-urlencoded";
request.ContentLength = byteArray.Length;

Stream dataStream = request.GetRequestStream(); // тут падаем с ошибкой - The request was aborted: Could not create SSL/TLS secure channel.