
Talk about the fic. If you care enough to tell the author they should tag something, write a normal comment about how you really like the fic first. Then, at the bottom of that comment, go “hey, I noticed you didn’t tag X, is there a reason it’s not tagged?” If you don’t like the fic enough to write a normal comment about it, it’ll come off as really really rude to just say “hey I want this tag added”.