I know that I'm so close to the answer, but I can't quite get there.
I want to give the player an item with the effect that the spell DC for WIS and CHA spells goes up by one. I can increase the global spell DC, but I haven't figured out how to narrow just to specific attributes. This post is how I learned the attribute key for the former, but I can't make the jump to my specific use from there. Any help is appreciated.