popup lightdismissal in windows store

Wrirte this code in Mainpage.xaml.
< Popup x : name = popup IsLightDismissEnabled = true >
< Popup . ChildTransitioins >
< TransitionCollection >
< PaneThemeTransition />
< /TransitionCollection >

<Border x : Name = PopupBorder Thickness = 6 Background = "#FF72A4EC"> < TextBlock x : Name = "tb" Text = "This is a popup with lightdismissal" />
 </Border>
< /Popup>  


Write this code  in Mainpage.xaml.cs.

if (!Popup.IsOpen)
                {
                    PopupBorder.Width = 658;
                    PopupBorder.Height = 100;
                    Popup.HorizontalOffset = Window.Current.Bounds.Width - 658 ;
                    Popup.VerticalOffset = 90;
                    Popup.IsOpen = true;

                }