Sorry, that is not a setting we currently offer.
Our philosophy for Fandom communities is one of openness and community editing, as noted on our Community Creation Policy.
While you're welcome to have a wiki for smaller group projects, we don't restrict access to them from other users.
However, we do offer the ability to disable the ability for unregistered users to edit. The wiki will still be viewable by everyone, but only those who have Fandom accounts will be able to edit.
If you are the admin of a wiki, you can find "Require all contributors to log in" as a switch on Special:AdminDashboard on your wiki. For more info about Admin Dashboard see here.
(Note: if the switch is missing, your wiki already has unregistered editing disabled.)