public partial class Form1 : Form
    {
        public Form1()
        {
            
            InitializeComponent();
            try
            {
                MessageBox.Show(Send(new System.Uri(@"https://172.16.1.3:10000/testResponse.asp"), "HELLO", ChoiceCert()[0], Encoding.Default));
            }
            catch (System.Exception e)
            {
                MessageBox.Show(e.Message);
                MessageBox.Show(e.StackTrace);
            }
        }

        public X509Certificate2Collection ChoiceCert()
        {
            X509Store store = new X509Store("MY", StoreLocation.CurrentUser);
            store.Open(OpenFlags.ReadWrite);
            X509Certificate2Collection storecollection = store.Certificates;

            X509Certificate2Collection certInspCol = X509Certificate2UI.SelectFromCollection(storecollection,
                                                "Choice cert",
                                                "Choice cert",
                                                X509SelectionFlag.SingleSelection);

            X509Certificate2Enumerator en = certInspCol.GetEnumerator();
            en.Reset();
            if (!en.MoveNext()) return null;
            return certInspCol;


        }


        public static string Send(Uri uri, string question, X509Certificate clientCertificate, Encoding encoding)
        {
            if (null == uri)
                throw new ArgumentNullException("uri");

            string answer;

            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(uri);
            request.ProtocolVersion = new Version(1, 0);

            if (null != clientCertificate)
                request.ClientCertificates.Add(clientCertificate);
                        
            //ServicePointManager.CertificatePolicy = new MyCertificateValidation();
            if (!string.IsNullOrEmpty(question))
            {
                request.Method = "POST";
                request.ContentType = "application/x-www-form-urlencoded";

                StreamWriter streamWriter = null;

                try
                {
                    Stream reqStream = request.GetRequestStream();
                    streamWriter = new StreamWriter(reqStream, encoding);
                    streamWriter.Write(question);
                }
                finally
                {
                    if (null != streamWriter)
                        streamWriter.Close();
                }
            }

            WebResponse response = request.GetResponse();
            StreamReader streamReader = null;

            try
            {
                streamReader = new StreamReader(response.GetResponseStream(), encoding);
                answer = streamReader.ReadToEnd();
            }
            finally
            {
                if (null != streamReader)
                    streamReader.Close();
            }

            return answer;
        }        
    }

<%@ EnableSessionState="False" Language = "JScript" CODEPAGE="1251" %> 
<%
      Response.Write("OK");
%>

WebResponse response = request.GetResponse();