Once you've made a booking with Transpennine Express you'll be sent a booking confirmation email.
After 24 hours, you'll be able to enter the confirmation number from the email (highlighted below) into the Seatfrog app and discover if upgrades are available on your trip. If you don't want to wait, try using the train search feature to add the trip to your account.
Please note that TPE services are added to the Seatfrog app each week and may not be available in advance.