From Xojo Documentation
Read-Only Property (As Ptr )
| This property is only available on Windows and Linux platforms.
For cross-platform compatibility, use ＃If...＃Endif with the Target... specifiers to make sure you will not attempt to use this property on an incompatible platform.
Returns the window handle that the TrayItem belongs to on Windows and the GtkStatusIcon widget on Linux. On Windows, the first TrayItem has a ID of 1000 and subsequent adds will increment this ID.