This works find by itself. xdotool windowminimize $(xdotool getactivewindow)
But not when used in an alias.
There are no windows in the stack
Invalid window '%1'
Usage: windowminimize [options] [window=%1]
--sync - only exit once the window has minimized (is not visible)
If no window is given, %1 is used. See WINDOW STACK in xdotool(1)