Spell - Targeted Unit

I ahve an ability based of firebolt that does 0 damage and 0 second stun. I want this ability to fire a trigger so i can select a unit to target witha a spell. How do i declare the targeted unit as this doesn't work:

Events
Unit - A unit Begins casting an ability
Conditions
(Ability being cast) Equal to Book (BlueBlack)
Actions
Unit - Order (Casting unit) to Stop
Unit - Change ownership of (Targeted unit) to Neutral Passive and Change color

Targetted unit is an event response for Event - Unit acquires a target.

You'll need to do something like they suggest in This Thread

Set the target to a variable.

Events
Unit - A unit Is issued an order targeting an object
Conditions
(Issued order) Equal to (Order(firebolt))
Actions
Set Target1 = (Target unit of issued order)

Thanks, didn't think it was possible with one trigger.

HOLD IT!

That will happen if somebody issues an order - You can issue an order from halfway across the map!

For your Event use "starts the effect of an ability" or "finishes casting an ability" if it isn't a channeling spell, that will prevent exploits.