Shift + Click over Unitframe
 

I am trying to get a small script done where I can define a spell that is cast when over a Unit frame and using Shift Left Mouse Click. But I am stuck it doesn't work so far. Basically something I could do with Clique but I just need it for 2 Spells and would like a more lightweight solution for this. Basically the approach would be:

WorldFrame:HookScript("OnMouseDown", function(self, button)
if(button=="LeftButton" and IsShiftKeyDown() and UnitExists("mouseover")) then

Yeah here I am stuck tried several things .....

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Firstly, why are you using WorldFrame instead of the unit frame? Secondly, casting spells is a secure function and will need more than what you've started with. I've never worked with it myself, so I won't try to give any help which would most likely be wrong. :)