Applying MandatoryOnTransition to subject field

I’m setting up 5.0 and I want to make the subject field mandatory. The MandatorySubject extension throws errors when I try to install it so I’m using MandatoryOnTransition instead. I have specified several fields to be required and they are working except for the subject field. I may not be using the correct field name. How do I find out what the internal names are?

Here’s what I have in my config in case there’s an obvious problem.

    'YLN' => {
        '__CREATE__ -> open' => ['Subject', 'CF.TicketSource', 'CF.YLNTopic', 'CF.YLNLibraryMakingRequest'],
    'YFL' => {
        '__CREATE__ -> open' => ['Subject', 'CF.TicketSource', 'CF.YFLTopic', 'CF.YFLLibraryMakingRequest'],

Seems like Subject isn’t listed as a Basic field supported

:person_facepalming: I should read the documentation first.