Get a hero to cast custom abilities through triggers?

Ok, this one is kinda complicated... At least I think so... :\

I got a trigger set so that under the right situation
a unit will be created and a custom ability given to it.
Now I want that new unit to cast the ability at
another unit.

The ability HAS to be given and can't be preset in the unit,
as it will vary depending on the situation. I see in the triggers
I can tell a unit to cast default abilities, but how do I get them
to cast custom abilities? Does it involve importing the custom
abilities?

Any help will be greatly appreciated. thnx in advance.

Just use action: Issue order targeting a unit, choose the order off the BASE spell there, that action reffers to the spell order string, wich does not change(unless you make the spell based on channel, changing channel order string works), so, if your spell is based on Storm Bolt, use:
Unit - Order (your unit) to Human Mountain King - Storm Bolt (target unit)

ok that is sorta insightful thnx.

but it dosen't solve the problem. I need the trgger cast spell
to be variable.
Basicaly This is what I want:

Player1 starts channeling Mimic ability.
If any spell directed at Player1 is cast within range,
A dummy unit is created and givin the ability being cast.
Then through triggers the dummy unit casts the ability
back at the enemy caster.

So I need a way to log the ability and cast that ability back
It will be the only ability the dummy unit will have. is there a
way to tell the dummy unit to just cast a random ability he has?