Hello Britt2477
It is not possible to purchase the Swiss Half Fare Card in the app.
See the following links to purchase the Swiss Half Fare Card.
If you purchase from SBB, it is recommended to create a user account first; by doing so, the Swiss Half Fare Card will be displayed in the SBB Mobile app, as an e-ticket. It will not appear in the app, and cannot be displayed when purchasing from STC - Switzerland Travel Center.
From SBB:
https://www.sbb.ch/en/billette-angebote/billette/gaeste-ausland/swiss-halffare-card.html
From STC:
https://switzerlandtravelcentre.com/en/che/ticket/swiss-half-fare-card
How to change passenger data to take the half fare into consideration, I described here:
https://community.sbb.ch/d/26418-how-can-i-save-the-info-of-swiss-half-fare-card
Please let us know if you have got more questions.
Kind regards,
Roland