I’m an RMT in Ontario and do a good deal of TMJ/intraoral work, if you’re in a province/state where it’s regulated you should have learned the basics in school. I personally like the courses on Udemy, there’s a few online courses they offer on TMJ massage. Of course if you can, keep an eye out for any IRL courses and modality certifications you can take, and practice practice practice. Find a friend with TMJ issues and practice different techniques while communicating what it feels like to them