You need to add the fact that you want a half fare ticket to your passenger profile.
The HFC permits you to travel with half priced tickets. It is not necessary to purchase half price tickets. The discount is thus not applied automatically.
I suggest you buy the Saver Day Pass here: https://sbb-tickets.cc/en/buying/pages/kontingent/kontingent.xhtml
Where you enter the passenger details you indicate that you have (or will have ) the Half Fare Card.