User user = new User();
        TextBox userName_txtb = (TextBox)this.Page.FindControl("username");
        user.UserName = userName_txtb.Text;
        TextBox password_txtb = (TextBox)this.Page.FindControl("passwordTextBox");
        user.Password = password_txtb.Text;
        TextBox email_txtb = (TextBox)this.Page.FindControl("email_link");
        user.Email = email_txtb.Text;
        TextBox telephone_txtb = (TextBox)this.Page.FindControl("mobile_number");
        user.Telephone = telephone_txtb.Text;
        TextBox firstName_txtb = (TextBox)this.Page.FindControl("f_name");
        user.Email = firstName_txtb.Text;
        TextBox lastName_txtb = (TextBox)this.Page.FindControl("l_name");
        user.Telephone = lastName_txtb.Text;
        TextBox birthdate_txtb = (TextBox)this.Page.FindControl("birth_date");
        user.Birthdate = birthdate_txtb.Text;

        // ..etc i fill the user object with values from the Registration form

        var webClient = new WebClient();
        DataContractJsonSerializer ser = new DataContractJsonSerializer(typeof(User));
        MemoryStream mem = new MemoryStream();
        ser.WriteObject(mem, user);
        string data = Encoding.UTF8.GetString(mem.ToArray(), 0, (int)mem.Length);
        webClient.Headers["Content-type"] = "application/json";
        webClient.Encoding = Encoding.UTF8;

        try
        {
            // here is where the exception is thrown
            **String str = webClient.UploadString(BASE_URL + "Create", "POST", data);**
            if (str.Equals("false"))
            {
                Session["error"] = "username already exists";
            }
            else
            {
                Session["userId"] = user.UserName;
                Session["login_status"] = "OK";
            }
        }
        catch (WebException ex) {
            printException(ex);
            throw ex;
        }

[OperationContract]
        [WebInvoke(Method = "POST", UriTemplate = "Create", RequestFormat = WebMessageFormat.Json, ResponseFormat = WebMessageFormat.Json)]
        bool create(UserServiceClass _user);