Preference > Mission Control > "K...Shortcuts" > "Show Desktop" field | Regardless of user choice, the field value resets to blank after restart.

The key assigned by user for "Show Desktop" field would reset itself to the default automatically, i.e. blank field (no keys are selected) after some time (possibly after rebooting). I tried to set "F1", "F2", "F5" etc. In the past, but the same behavior would recur.

Note: The other 2 spinboxes "Mission Control" and "Application Window" would however retain the initial value.

My hands are kinda too big to readily/easily spread with thumb and three fingers on the MacBook Air trackpad.

The field can be chosen from any Keyboard keys listed as below (the [Shift] and [Command] modifiers is not needed usually, I had to use both modifiers to activate screenshot function.)

Just in case, the page containing the alleged component can be accessed as below.

1. 1. 1. Activate "System Preference"
      2. Select "Mission Control" menu.
      3. Locate "Keyboard and Mouse Shortcuts" section.
      4. Locate the third spinbox field with "Show Desktop" text labeled on its left.

Selecting the spinbox button would prompt a scrollable list of Keyboard keys that user can assign as shortcut key to trigger "Show Desktop" action, that displays the desktop background while changing focus/control to desktop interface after some animation.