Oh, that's another story.
The problem is, that most admins are arrogant jerks, who fancy themselves too important, because they are running a forum (like that would be so special) and they can't differentiate between unrelated spam and shared interest.
They probably also think, if they let you post a link for free to your
topic related app or whatever, they will lose millions and billions of dollars on their advertising plan even if their website never had any advertisement sold.
Don't get me wrong, I am not for spamming. Absolutelly not! I believe in what I am doing that's why I am doing it and I am often annoyed by the stupidity of people, who can't see, if you are a "rush-in-rush-out" spammer, who links any spam on any unrelated forum, or you are somebody, who tries to make people with the same interests aware about something they might be interested in.
The funny thing is, they don't let you "advertise" your thing, that could be helpful for their userbase and maybe it even would do something good for them too, but they let every scam coming through on google adwords and even pray for it being clicked by their precious users, which they try to protect so hard.
/rant OFF
So you have to trick a bit to bring people joy and happyness
If you want to advertise your app for your target group, you have to participate as a totally stranger. I mean, register and post a few posts and then let the link slip through like "hey guys, look what I have found".
Most admins have not even enough IQ to notice or they will be just unsure about it and if you have luck, the link can remain.