App.xaml.cs

        protected override void OnHardwareButtonsBackPressed(object sender, Windows.Phone.UI.Input.BackPressedEventArgs e)
        {
            var page = (Page)((Frame)Windows.UI.Xaml.Window.Current.Content).Content;
            if (page.GetType() == typeof(FirstRunPage))
            {
                var firstRunPage = (FirstRunPage)page;
                if (firstRunPage.CanGoBack())
                {
                    e.Handled = false;
                }
                else
                {
                    e.Handled = true;
                }
            }
            base.OnHardwareButtonsBackPressed(sender, e);
        }