I dunno, but https://www.usa.gov/write-in-candidates
Check with your state election office to find out the rules for your state. If you check using your state's election website, enter "write-in candidates" in the search bar.
It may be better advice to ask a local PSL representative what is the right way to do things if you want to vote for them, because the official government channels where you would think to inform yourself are probably 100% controlled by either the Democrat or Republican party and may intentionally give out misleading or confusing information in order to get people to not vote or accidentally invalidate their ballots so that third party candidates get fewer votes.
