The User ID is, as the name itself says, a unique identifier associated with each account. This ID is generated when you first create your account with one of our games and it remains the same while the account is active, serving as an identification of the user.
This also helps us track any accounts, which will ultimately make sure we can more quickly and more efficiently help you resolve your issues.
On the majority of your games, you will find your User ID by clicking on your Profile Picture which will bring up your game profile statistics and there is where the User ID is.
Below you can see a screenshot of where you can find your User ID: