mb99 You must buy a ticket with this possibility for every trip, search a timetable on sbb.ch or in the SBB Mobile app and the system displays by default the half fare prices. You must indicate that you have this discount when you define the passenger.
I forgot something. If you want to buy this Half Fare Travelcard, you could also ask at the ticket office for the refunds of your tickets (but, as indicated earlier, a refund is only possible before the day of validity).