If you create an account on www.sbb.ch, use it to purchase the Swiss Travel Pass, and then log in to your app using the same account the STP will be there automatically.
If you bought the STP as a guest, or bought it from some other website than SBB you cannot add it to the app afterwards.
But you do not need to add it to the app, as all you need to be able to do is show it to a conductor. So print it out. Store a copy of the PDF on your phone. Show it when asked.