Hey David,
This doesn't work on either my Mid-2010 17"MacBook Pro running Sierra or my Mid-2012 MacBook Air running Mojave.
The external keyboard on the MacBook Pro will fire when the actual Enter key is pressed – but no other “equivalent” keyboard shortcut works.
Interestingly – I set the hotkey using fnReturn...
I suspect there's something funny going on at the hardware level.
See this thread:
If that is unsatisfying search for:
“fn key” on the forum.
If you're still unsatisfied then contact support.
Contact Keyboard Maestro Support
-Chris