Hello johnsonng
International tickets cannot be purchased through the app, you have to buy them through the website www.sbb.ch. To claim the reduction with Swiss Travel Pass, choose GA Travelcard as “Discount card”.
For national trips you do not need to get tickets, the Swiss Travel Pass is your ticket. Simply board the train with your STP. You must present your physical passport (no copies) together with your Swiss Travel Pass upon ticket check.
Kind regards,
Roland