Are you selecting a model before calling the mapkey?
I tried the same here and the mapkey worked fine.
mapkey test ~ Timer `UI Desktop` `UI Desktop` `popupMenuRMBTimerCB`;\
mapkey(continued) ~ Close `rmb_popup` `PopupMenu`;~ Command `EMX_MOD_COMP`;
Or maybe some problem in your application.
I made a short video:
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Good luck,
Jose