Order unit to cast custom spell in trigger

This is quite a noob question, but I can't seem to find a way to order a unit to cast a custom ability in trigger, like Order (Unit) to Something. I've noticed that there's the Order String stuff in the Object Editor, but I don't know how to work it, like what to specify for the trigger editor to recognize it. Is there a way to cast a custom ability the way you would a melee one?

You order the unit to cast the spell your custom spell is based on. So if you created a spell based off of Storm Bolt, then you'd do this:

And if the ability is not in that list?

All abilities should be there. Check all the categories: Melee, Campaign, and Custom.