Both are correct and can be used from what I understand, but I'm not a native speaker. The context of the sentence itself may be relevant I think. I agree with knyte that "at" could also work in some cases, when you think about "over there, at the forum" as a distant place. For example: people have been written things "at the forum".
Also, I have absolutely no clue why you would ask such question!
Do you say on the forum or in the forum?: 8/8/2018 03:20:00