Sometimes you can outsmart yourself.
I looked at this code, and thought, "a-ha"

. I'll use mar3us20, since I'm in the US, and that's how Kobo usually does its codes.
So, I did, and was BUMMED that it didn't work. I tried two or three books (all non-agency) and it didn't work on any of them.
So, then I thought, WTH

, I'll just try the code as-is, with the "ca" in there.
SUCCESS!