FVWM: Add button 8 on title bar, press to "always on top" or cancel

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Hi,
I am a newbie, so always have questions. Thanks for your help. :-)

I want to add  button 8 on title bar, and  want press it to make the
window always on top and press again to cancel always on top. How
can I do that. I have read fvwm manual, and can not find any solution.

Thanks.