Can't figure out how to add the passive ElementalAffinity_extraDamage to my custom class

[Mythrialus]

I'm trying to make a new Sorcerer class that is similar to the Draconic Bloodline Sorcerer, but uses Constitution instead of Charisma as the spellcasting ability. I've managed to change the spell casting ability and add the elemental resistance, but I can't seem to figure out how to get the passive that adds your spellcasting modifier to your elemental damage at level 6 to work. It will add it as a To Hit modifier, but it doesn't seem to add anything to the damage, as during my tests, I'm still coming up with low numbers that shouldn't be possible with a high modifier. Can anyone help????