public async Task<string> RegisterUser(UserLoginModel loginModel, string authenticationToken)
        {
            if (!loginModel.IsValid())
            {
                return "invalid_model";
            }

            using (var client = new HttpClient())
            {
                var formContent = new FormUrlEncodedContent(new[]
                {
                    new KeyValuePair<string, string>("Username", loginModel.Username),
                    new KeyValuePair<string, string>("Password", loginModel.Password)
                });

                client.DefaultRequestHeaders.TryAddWithoutValidation("Authorization", "Bearer " + authenticationToken);
                var postResponse = await client.PostAsync("http://resz.azurewebsites.net/User/createUser", formContent);
                postResponse.EnsureSuccessStatusCode();

                return await postResponse.Content.ReadAsStringAsync();
            }
        }
        async partial void btnRegister_TouchUpInside(UIButton sender)
        {
            lblRegisterFormFeedback.Text = "contacting API, please wait ...";
            lblRegisterFormFeedback.TextColor = UIColor.Blue;

            UserLoginModel model = new UserLoginModel(txtRegisterUsername.Text, txtRegisterPassword.Text);

            if (!model.IsValid())
            {
                lblRegisterFormFeedback.Text = "please fill in your details";
                lblRegisterFormFeedback.TextColor = UIColor.Red;
                return;
            }

            ServiceWrapper serviceWrapper = new ServiceWrapper();

            TokenModel tokenModel = await serviceWrapper.GetAuthorizationTokenData();
            string authenticationResult = await serviceWrapper.RegisterUser(model, tokenModel.access_token);

            lblRegisterFormFeedback.Text = authenticationResult;
            lblRegisterFormFeedback.TextColor = UIColor.Blue;
            
        }