Your Player/User ID (which is often denoted as UID) uniquely identifies your account and purchase history. It also speeds up troubleshooting in support - especially for missing items or billing issues.

If you contact us through the app by going to Settings > Open Email, your player UID is automatically included in the prepopulated email. 

Alternatively, you can find your player/user ID on the home screen of the game by clicking on the account menu item with the shiba button icon in the menu on the top-right.

Your player/user ID is the first item there, you can press the copy button to copy the UID and paste it into your support reports/queries.

More instructions here.