Are you trying to advertise your forum? If your forum is new and you are trying to get people to sign up I would say that is a bad idea but If you have services you are trying to promote then use Google AdWords.
I personally would not recommend doing so. You can try https://forumpromotion.net/forum/ as a start, try also postloop to help you kicking off discussions/contents and make it look active. Be patient we all have been there when we first started. Use soical media esp fb and tw to link back to your forum.
Even if you did run ads on another forum it would only make sense to have them on forums within the same niche, and these would be your competitors. Are they likely to want that ?