INT 16 - PC Tools v5.1+ DESKTOP API - DISPLAY POPUP MENU
AX = FFDEh
DS:DX -> menu description (must be on a paragraph boundary)
Return: AX = ???
AL seems to be the number of the selected button
Note:   available only when popped up

See also: 16FFEE

16FFDE - PC Tools v5.1+ DESKTOP API - DISPLAY POPUP MENU