Almost every forum will allow a link, but that is only for the established members. If you find one that allows instant links, then there is a likelihood that they are spammed to death, and they are mostly unmoderated. This leaves you with a source of poor quality links. Just focus on building a good reputation in good forum then when you have earned enough authority and reputation by sharing good quality content, you can start sharing links. And again, nobody would appreciate you over-marketing yourself and not being helpful. The aim of having forums is to share and discuss with others, promotion only comes as an AOB.