A Question

I want to make a unit cast a spell when another unit begin chanlling another spell.
How should I write the trigger?

I believe there is an event for "begins channeling a spell".

I knew that.
But I can't find a action to ORDER the unit to cast a CUSTOM SPELL!
So I suppose the JASS can help me.
But I know little about JASS.
Then I post this thread.

You can order a spell cast with the standard Issue Order ... . Just use the order string of the spell you have based it on.