when you finish building your roster, choose a team name and ballpark. then click create team. you'll then get a menu asking about invitations to the league. you can either send invitations or skip that and choose advance to team center.
once that's done, the league number will appear. it shows up in the team center along with your other teams. there;s no waiting for WIS to assign you a number, if you're done it correctly the number shows up immediately.