Hello TDW
The Half-Fare Card is only considered in the app from the first day of validity. However, you can adjust your details in the settings under Travelers. Simply select the Half-Fare travelcard in the subscriptions and buy the tickets in advance. However, if you buy tickets with departure before the validity of the Half-Fare travelcard, I ask you to delete the manually added travelcard again, so that it does not come to a wrong purchase.
Thank you for your understanding and have a nice day.
Greetings,
FatimaH