I almost suspect your comment is spam, meant to promote the site you linked to. If not, explain what you mean by “signinless”? Basically, the way forums work is that you generally have a username. Thus, you have to sign in somehow, whether that’s integrated w/ FB, Google, etc., or via a standard username/password. As for the former, there are plenty that allow that and yes that could be custom scripted. But, you could search for discussion forum software that allows FB sign-ins, for example. Should be fairly easy to identify. (I don’t know many off-hand, as I try to avoid working on forums, as described in this article.) Good luck!