If you're talking about convention and etiquette, there's not too much difference between here and other social media (except that longer posts are welcome). I guess I'd liken it to a YouTube comment section: You can start your own discussion, or you can reply to someone else's discussion, and @ people you specifically want to address.
As others have already stated, there are tons and tons of promotional threads being created on the hour every hour, so if you just want places to advertise you can totally just hop into one occasionally.
HOWEVER: I think you'll get better results if you just use the forum like a regular person and actually talk to people about art. You'll find plenty of opportunities to bring up your work and your characters and the kinds of stories you tell just by participating in real discussions...and since this forum is obviously 90% creators and 10% consumers (if that), I think that's the best way to use this forum for promotion.
Most of the people here aren't starving for content and just waiting for someone to advertise the right genre buzzwords at them; they're passionate storytellers themselves, and will respond to people who can show that they share that passion.
And honestly, I feel like this forum would be a much more interesting place if more people took the time to do that instead of just dropping links and leaving. There are actually still a lot of people here; this forum is VERY active, but it doesn't feel like it because most of the users don't talk outside of the Promotion category. Any discussions that AREN'T focused on advertising seem to live or die by the same 20 or so regulars...it's such a shame. You would think that a site with such a large percentage of creative people on it would feature an equally large percentage of discussions about...well, creation. Instead it feels like the opposite...