The Custom League URL is created by the league commissioner during registration, or at any time during the season by using the Commissioner Tools. This URL may be changed at any time and can be carried over from one season to the other.
If you're not sure what this URL is, you can either contact your commissioner or view it in the League Settings.
This URL can be bookmarked and may be easier to remember than your League ID#. Any time the URL is changed by your commissioner, email notification will automatically be sent to the league and a notice posted in the Recent Transactions.