Is it even possible to advertise a forum on Reddit or the like without looking like a huge shill?
You bring it up in casual conversation on a popular thread related to adventure writing - /r/DnDBehindTheScreen, /r/dungeonprompt, /r/UnearthedArcana , r/RPGdesign, r/AskGameMasters, r/DMAcademy, and r/DndAdventureWriter are probably the most receptive candidates.
You go into one of those "Hey GMs, how do you come up with adventure ideas"-type threads, and post something like "Well here's what I do blah blah blah... Oh, and it also helps to bounce ideas off other guys, for instance I use the tenfootpole forums to check with other game masters if I think an idea is good, etc." then some guy's all like "what forum?" and you're like "Ah, you heard of tenfootpole's reviews, right? Well there's a forum there with a bunch of designers and... BAM! HERE'S A LINK, BITCHES!" and people are all like "Ooh! Industry writers! Neat!"
It's not like we're selling something to them. It's not an advertising campaign, and we aren't looking to push product or collect money. We're just linking to a forum, same way how nobody shits their pants if you added "...well there was this conversation about that on dragonsfoot the other day..." into your post.
So long as you also put your regular input into the thread that you were going to do anyway, it's not so much shilling as it is just linking to somewhere else on the interwebs.