Hey Greg,
Firstly – welcome to the forum! 
@Sleepy is correct to ask if you've looked through all the menus to make certain there's not a menu command available.
Assuming you've done your due diligence on that...
Keyboard Maestro doesn't have a button detector, and its press a button action can't see into every UI widget on macOS.
When doing detective work on the macOS UI I use UI Browser, Script Debugger, and sometimes the Accessibility Inspector.app from Xcode.
Personally I'd try to use AppleScript UI-Scripting to do this, but that's a row to hoe for most folks.
Here's a place to start with that:
Front Window Analysis Tool Using AppleScript System Events (List UI Elements)
-Chris