//save this file to the ZBrush 4R7\ZStartup\Macros\Misc folder and press Macro>Reload All Macros

[IButton,???,"Use hotkey to place alpha",
    [VarSet,lm,0]
    [If,[IGet,Stroke:LazyMouse],[Varset,lm,1][ISet,Stroke:LazyMouse,0]]
	[CanvasClick,[MouseHPos],[MouseVPos],[MouseHPos],[MouseVPos]-2]	
	[If,lm,[ISet,Stroke:LazyMouse,1]]
,,.5]