Have a look in Tasma > Peripherals > Keyboard Shortcuts, Keyboard or Keyboard Layout.
I expect the first (Keyboard Shortcuts) item.
Nope, can't find it there! It's just for "normal" special keys like alt, windows, ctrl and so on, but not the "very special" keys. It's the ones you cannot just apply to a program when pressed because they are out of normal keycode range. You only get Linux to react to them with xev in a terminal - or with a special daemon/progam like hotkeys or lineakd.
Any other ideas?