I'm not aware of anything on LW, but on reddit there's r/HPMOR for just HPMOR and r/rational for ratfics in general.
There's not a special forum for it. You can either comment on individual HPMOR chapters on the site, or make a new top-level post of an appropriate type. (i.e. make a question for questions, a personal blogpost if you have a lot to say and think about, or perhaps a shortform post if you have something quick to chat about)
Hello. I'm new to LW. I was wondering if there's a forum to ask questions and talk about HPMOR?