Is there any way to add a cantrip to a homebrew subclass that they may not usually get? If so how would I do that? Thank you!

Depends on which class the subclass is for. I know, that sounds strange, but it’s true.

How do we know which classes this works for and doesn't?

Technically it’ll work on all of them, it’s just a matter of how you want the Cantrip “added” which determines how it has to be accomplished. Do you want to add it to the list of spells from which they can choose, or do you want it added as a “free” extra spell?

It was added as a free extra cantrip!  I put everything in right but it wasn't loading properly but then I shared the character sheet and it decided to work...

Thanks for your help!

Gotcha. Yeah, what likely happened was th character sheet fell asleep with an old cache and by having the link generated it triggered it to wake up and refresh to the new lest update you made. I don’t know how much I really helped, but I’m glad it’s working for you.