AI Cast Custom Spell: Custom Script Help

[All_Time_Pro]

Pharoah posted this a long time ago when explaining how to do custom scripts for "How to Make AI cast a Custom Spell" in a previous thread:
How To Make An AI Cast An Custom Spell

I need a unit that is not already on the map to cast a custom spell on an enemy unit that is in range. If the unit is not made yet and it will get its own variable how to do I enter in that Variable name for the "yourunit" part and what do I put for the "targetunit" part if the unit is based on being in ranged and being in range. I understand that the custom ability must be done with custom script but I really need help identifying the missing pieces to make the trigger.
The custom spell is a breath of sand ability from the Giant Sand Worm model by Sephiroth_VII it matches the "Unit-target type abilities" that Pharoah described.

Thanks in advance!

 

[All_Time_Pro]

Note: I just finished the trigger myself but not at all with custom sciript... Still curious as to how I to use it the way Pharoah explained.