Capital Letters on Form

Hello, on my form I have a box where I write a reference, wither FA000000 or FAM00000000 (so two letters followed by six numbers, or three letters followed by seven numbers).

I want the two or three letters to automatically become capital after they're entered?

People have recommended putting coding in After Update in Event, but I don't know what to write. I tried >LL/000000;;_ in the Input Mask, but is useless when I want to use the longer reference.

Thanks for any help.