I dunno, maybe a stickied thread for that.
Really your RP is it's own advert (when you post the OOC and finally the main thread).
Another way is to give the ad to Rogue47 for his weekly thing, and you can tack a link and description in your signature (which is the most common effort).

Can always first try an interest check like lots of people do - this will get some preliminary interest sparked.

Lot's of ways other than a sub forum (which is a bit much and will basically just be the first posts of OOC's).

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