what is the syntax for using the 'opener' tweak for a file?

Windows 10, Calibre 8.3.0

I am testing using a different Windowa reader for some files (rather than using the default or the calibre viewer) but it only opens the windows default (Sumatra in my case).

As a test, I have deselected "EPUB" in <preferences / behaviour> settings.

I attempted the following permutations:
openers_by_scheme = {'file': 'Acrobat.exe %u'}
openers_by_scheme = {'file': 'C:\\Program Files\\Ado ,...snip...> at\\Acrobat.exe %u'}
openers_by_scheme = {'file': 'C:\\Program Files\\Ado ,...snip...> at\\Acrobat.exe'}


I also tried these (with full pathname as well)
{'epub': 'Acrobat.exe %u'}
{'EPUB': 'Acrobat.exe %u'}

What is the correct syntax / command to have a different reader open the file?