For free? You can post on the service board in search of some volunteers who're not currently monetizing their signature space to wear your signature ads for free, you're likely going to get some volunteers that would wear your signatures if truely what you offer can be proven to be authentic. This ad will be display as they engage in conversation on the forum.

Interested in paid ad?, The forum can help with that, it offers ad slots in form of auction that if you win, you'll given some number of slots to advertise your project which will be displayed at the top of all tooi or you can make use of paid signature advertising.

Ad slot: https://bitcointalk.org/index.php?topic=5244598.0
Service board: https://bitcointalk.org/index.php?board=52.0