[DllImport("User32.dll")]

 public static extern IntPtr SetFocus(IntPtr hWnd);

 IntPtr GetHwnd(Popup popup)

 {

     HwndSource source = (HwndSource)PresentationSource.FromVisual(popup.Child);

     return source.Handle;

 }

 private void ShowPopupButtonClick(object sender, RoutedEventArgs e)

 {

     ThePopup.IsOpen = true;

     IntPtr handle = GetHwnd(ThePopup);

     SetFocus(handle);

 }