Steam - "This Xbox Live Account is already used with another Steam account on Microsoft Flight Simulator" error message

This message appears when you log into Microsoft Flight Simulator for the first time on a different Steam account than the one used to purchase the game or from your account, but with a different Gamertag. 

CONTACT SUPPORT

To unlink your account, please contact our support team.

1. Click on Submit a request
2. In the drop-down menu select I receive the error message "This Xbox Live Account is already used with another Steam account on Microsoft Flight Simulator

Your ticket must include the following information:

1. Your gamertag
2. The Steam ID64 of the Steam account that is incorrectly linked
3. The Steam ID64 of the Steam account you want to link instead

To learn how to find your Steam ID, visit "Find your Steam ID64 before contacting support"