Issue Custom spells by triggers.

How do I set up a trigger that will tell a unit to cast a custom made spell? Normaly I would just tell them to cast what the base spell was. but in this case the base spell is an item ability. The ability is from the "Scroll of Speed". so its targetless.

Just do 'order unit to <the base spell for your custom spell> targeting a point/area/no target'(depending on your spell.

Just go to ability u want and change order string to [order string u want if it wont have like some custom item] then use trigger to make em cast it for scroll of speed u can set order string to thunder clap then make an action like that
Order with no target-order unit cast thunder clap
Hope it help

That won't work, you can't just change spell's orderstrings. I had a similar problem, here is the solution.