How to insert data to Mobile Services - Windows Store App (Windows Azure)

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using Windows.Foundation;
using Windows.Foundation.Collections;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Controls.Primitives;
using Windows.UI.Xaml.Data;
using Windows.UI.Xaml.Input;
using Windows.UI.Xaml.Media;
using Windows.UI.Xaml.Navigation;

using Microsoft.WindowsAzure.MobileServices;
using Newtonsoft.Json;
using Windows.UI.Popups;

// The Blank Page item template is documented at http://go.microsoft.com/fwlink/?LinkId=234238

namespace myApp
{
    /// <summary>
    /// An empty page that can be used on its own or navigated to within a Frame.
    /// </summary>
    /// 
    public class MyMember
    {
        public int Id { get; set; }

        [JsonProperty(PropertyName = "username")]
        public string Username { get; set; }

        [JsonProperty(PropertyName = "password")]
        public string Password { get; set; }

        [JsonProperty(PropertyName = "name")]
        public string Name { get; set; }

        [JsonProperty(PropertyName = "tel")]
        public string Tel { get; set; }

        [JsonProperty(PropertyName = "email")]
        public string Email { get; set; }
    }

    public sealed partial class MainPage : Page
    {

        private MobileServiceCollection<MyMember, MyMember> items;
        private IMobileServiceTable<MyMember> memberTable = App.MobileService.GetTable<MyMember>();

        public MainPage()
        {
            this.InitializeComponent();     
        }

        private async  void btnSave_Click(object sender, RoutedEventArgs e)
        {
            try
            {
                items = await memberTable.ToCollectionAsync();
                var insertItem = new MyMember
                {
                    Username = txtUsername.Text
                    ,
                    Password = txtPassword.Password
                    ,
                    Name = txtName.Text
                    ,
                    Email = txtEmail.Text
                    ,
                    Tel = txtTel.Text
                };
                await memberTable.InsertAsync(insertItem);
                items.Add(insertItem);

                // Dailog
                MessageDialog msgDialog = new MessageDialog("Register Data Successfully.", "Status :");
                await msgDialog.ShowAsync();

            }
            catch (MobileServiceInvalidOperationException ex)
            {
                throw ex;
            }
        }
        /// <summary>
        /// Invoked when this page is about to be displayed in a Frame.
        /// </summary>
        /// <param name="e">Event data that describes how this page was reached.  The Parameter
        /// property is typically used to configure the page.</param>
        protected override void OnNavigatedTo(NavigationEventArgs e)
        {
        }

        
    }
}