Order the unit to use spell, from which you based off your own spell.
In other words - if you based a spell off of Inner Fire and named it like Enrage (or something, it matters not), then you have to issue the unit to cast "Human Priest - Inner Fire".
If you were to base it off Lich's Frost Nova, then you have to issue "Undead Lich - Frost Nova"
If the spell is based off "Channel" spell, then you have to order the spell which you put in the Base order ID field.