Post-purchase it is not possible to link the Swiss Travel Pass with the SBB Mobile App.
In order to make the Swiss Travel Pass appear in the app, you must have bought the pass from the SBB webshop.
Furthermore, to buy tickets and view them in your app, you must have created a SwissPass account before purchase and being logged-in during purchase. Later you have to log-in in the SBB Mobile App using the same credentials.
How to display the pass if not from the SBB Mobile app? Store your pass as PDF on your mobile phone and show it as such to the conductor.