When searching for a new flight, an indicator will show against an airline where an unused credit could be used:
Click on ‘Show details’ for more information
When the booking has been completed, we have automation which checks the unused ticket database and applies these to the booking so tickets should not be wasted. If the value of the credit is higher, we store the balance to be used again for a further booking. This applies to all online bookings, you don't need to call in to use them.
If you explicitly want to ensure a specified unused ticket credit is used on a new booking, you can call the Amex GBT offline team who can process this for you using the phone number in the platform.