Why do you want to “use” it in the SBB app? What you got with your STP is an e-ticket basically consisting of a QR code, and this QR code represents some unique URL to your ticket record on the “SBB” server infrastructure. So a ticket control consists of scanning that QR code, and it’s completely irrelevant how (or on what) that QR code is displayed: Printed on paper, mail app, PDF viewer, whatever.
And if you want to buy discounted mountain ride tickets, simply proceed as per Roland’s link above. Whether you have a STP or some other half fare travelcard is NOT cross-checked during the ticket purchase process. You only need the STP only WHILE travelling with a discounted ticket (or at the cable car station) as proof of entitlement.