Auto Spell Casting

Hello !
How can I force a unit (tower) to cast a certain spell on an enemy when it enters the Casting Range of that spell (instead can be an integer equal to the casting range of that spell) ?
Bye Bye !
"I've created an idea about this but it seems to be a bad one so don't ask me to show that here"

Make the tower's attack range the same as the spell. When the tower attacks, make it use the spell.

OR, whenever the tower attacks, check if the target is in X range of the spell. If so, cast it.