... as a class feature.  The cantrip is homebrew if that matters.

Also there doesn't seem to be a way to cause a spell to cause a target to have disadvantage on opportunity attacks.  Is there a way around that?

You cannot specifically choose the stat but you can have it add the spellcasting ability to the roll. Modifier Type: Damage, Sub-Type: the damage type, enter dice roll info and there is a checkbox for "use primary stat'" - check this.

Quote from Cyb3rM1nd >>

You cannot specifically choose the stat but you can have it add the spellcasting ability to the roll. Modifier Type: Damage, Sub-Type: the damage type, enter dice roll info and there is a checkbox for "use primary stat'" - check this.

 Right but how do you have this modifier attach to a cantrip?  Kind of like the invocations that add damage to eldritch blast?

When you create the spell, after filling in the details and saving you get to add modifiers, higher level scaling and conditions. Click Add Modifier.

Sorry my brain glitched, it's a class feature.

There is no way to do this as a class feature.

Frustrating, but oh well...