By default, any registered user can post in a wiki forum (I assume you actually mean Discussions). It is only when you want to prevent users from doing to that you need to do something. To prevent non-registered users from editing/posting on the wiki, use the toggle that Headquarter8302 mentioned. If you want to restrict it to autoconfirmed users (registered users who have been registered for at least 4 days), you have to contact Fandom. That is not something you can do on your own. Again, the default permissions on a new wiki are already more permissive than what it sounds like you are asking for.
It is possible FANDOM turns this (preventing non-Autoconfirmed users) on by default and doesn't let you turn it off for some wikis... they do this for wikis aimed at kids by not allowing you to enable anonymous editing.