Your League ID can be used for a few different things. You can find it on both the web and mobile apps.
Once you're logged into Sleeper, select a league. Your League ID is located at the end of your URL. It's an 18-digit numeric string that looks something like this:
Go toward the bottom of your General League Settings. You'll see a spot to Copy League ID:
If you run into any trouble or have any questions, please email [email protected].