public partial class Form1 : Form
    {

        Back2uSubscribeManageService client = null;
        string certName = string.Empty;
        RemoteCertificateValidationCallback
        chainedServerCertValidationCallback = null;


 public Form1()
        {
            InitializeComponent();
            chainedServerCertValidationCallback =
            ServicePointManager.ServerCertificateValidationCallback;
            ServicePointManager.ServerCertificateValidationCallback = new
        RemoteCertificateValidationCallback(OnValidateServerCertificate);

        }

//below the method where I call a method of the web service

private void button1_Click(object sender, EventArgs e)
{

X509Store computerCaStore = new X509Store(StoreName.My,
         StoreLocation.LocalMachine);
    computerCaStore.Open(OpenFlags.ReadOnly);
    X509Certificate2Collection certificatesInStore =
    computerCaStore.Certificates;
    X509Certificate2Collection findResult =
   certificatesInStore.Find(X509FindType.FindBySubjectName,"*.mydomain.com", false);
            X509Certificate cert;
            cert = findResult[0];

client = new Back2uSubscribeManageService();
client.ClientCertificates.Add(cert);
client.Url = url;
var result = client.subscribeProduct(request);
}
}
//below is the callback method for server certificate validation.

 bool OnValidateServerCertificate(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
        {

            X509Store computerCaStore = new X509Store(StoreName.Root,
               StoreLocation.LocalMachine);
            X509Certificate2Collection certificatesInStore =
              computerCaStore.Certificates;
            X509Certificate2Collection findResult =

  certificatesInStore.Find(X509FindType.FindByIssuerName,"xxxxx", false);
            X509Certificate cert;
            cert = findResult[0];

           bool valid = true;
           if (valid)
            {
                if (chainedServerCertValidationCallback == null)
                {
                    bool validationResult =
                        certificate.Issuer.Equals(cert.Issuer);
                    return validationResult;
                }
                else
                {
                    return chainedServerCertValidationCallback(sender,
                   certificate, chain, sslPolicyErrors);
                }
            }
            return false;
        }
    }