How do I receive the discounted fare on the Bernina if I have a Swiss Pass?
I believe you just need to purchase a 10 chf reservation and show your properly validated Swiss Pass.
The Bernina Express route is fully covered by rail passes and a seat reservation fee is all you need to sit in the panorama observation coach. You can buy that in a train station or at https://www.rhb.ch/en/panoramic-trains/bernina-express. Plus a separate reservation fee if you will take the bus portion running Tirano-Lugano.