Your League ID can be useful for a variety of things, including troubleshooting, API use, and sharing league-specific information.
You can find your League ID on both web and the mobile app.
Web
Once you're logged into Sleeper, select a league.
Your League ID can be found at the end of the URL in your browser.
It will be an 18-digit numeric string, similar to this:
Mobile App
Open your league.
Go to League Settings, and open the General League Settings section.
Scroll down and at the bottom, you'll see an option to Copy League ID:
If you run into any trouble or have any questions, please email [email protected].