static guintptr video_window_handle = 0;
...

 if (video_window_handle != 0) {
   GstVideoOverlay *overlay;
    
   // GST_MESSAGE_SRC (message) will be the video sink element
   overlay = GST_VIDEO_OVERLAY (GST_MESSAGE_SRC (message));
   gst_video_overlay_set_window_handle (overlay, video_window_handle);
 } else {
   g_warning ("Should have obtained video_window_handle by now!");
 }
...

#ifdef GDK_WINDOWING_X11
  {
    gulong xid = GDK_WINDOW_XID (gtk_widget_get_window (video_window));
    video_window_handle = xid;
  }
#endif
#ifdef GDK_WINDOWING_WIN32
  {
    HWND wnd = GDK_WINDOW_HWND (gtk_widget_get_window (video_window));
    video_window_handle = (guintptr) wnd;
  }
#endif
}