Trying to assign 'show desktop' shortcut to f13

Sonoma. I want to have 'show desktop' as a one key action, like on my previous computer. I've been to System Settings and Keyboard, then Keyboard Shortcuts. I can change the shortcuts for screen captures if I want to, but I can't find anywhere to add in or change a shortcut for f13 or any of those.

I know I can use f11 for showing the desktop, but it can only be one keystroke if if I trade for two keys on volume, which is annoying as well.