I don’t think that’s possible since I have this issue with all my costume keyboards.
Some of them have only 36-keys. Such keyboard layouts do not exist in the Plasma keyboard settings.
They all use QMK as a firmware so I can program the keys to do anything I want.

I have not found any… I guess I would need to select: “Generic | Generic 121-key PC” as the keyboard model, which does not exist.
This is the keyboard: https://github.com/bluepylons/Boston

But it’s running QMK so one would have the same problem e.g.: with a 36-key keyboard which is programmed to send F13-F24 in some way…

https://github.com/bluepylons/Boston