An in-game ID is your unique user identifier within the game. The teststar support team might request your in-game ID to help investigate issues related to your progress or in-game purchases.
Please note that the in-game ID is specific to the game you're playing and should not be confused with your teststar customer number, as they are completely unrelated.
You can usually find your in-game ID in the game’s Settings. It may be referred to as "User ID", "Player ID", "Account ID" or something similar.