Adding a floating spell from another class

[Brolthemighty]

Alright, so my DM has given my a ring that grants a 1/short rest spell from the Warlock class list (am not a Warlock). But I can change out which spell it is, as long as it's of a level I can cast.

I thought creating a little selector like the Bard's arcane secrets on the ring would work. I got the ring to have a selector where I chose what class, but it wouldn't pull up any spells.

I would REALLY rather not have to create a whole new configurable/tab for a single spell selection and was hoping for a better solution (although if I must, I must). Anyone able to help?

Wondrous Item: Ring of Blending
Requires: Attunement and equipped
Grants:
- Alter Self CHA/long rest
- Floating Warlock Spell 1/short rest
- Spell can be of any level I can cast, and it's changeable.