The pass is your ticket.

How to book a reservation is explained here:

https://www.seat61.com/trains-and-routes/bernina-express.htm#How_to_make_a_Bernina_Express_reservation

You can also travel without reservations, since the front carriages of the Bernina Express don't have reservations. There are also many regular trains on the same route.

Okay, thank you. I just booked the seats. But how do they know I have a valid interrail pass? They did not ask the interrail number…

And last question, now that I bought the reservation, I can just jump in or have to do other things?

They don't need a pass number. The train staff will ask to see your pass and reservation and that's enough.

Before you board, you need to add the train to your pass in the app, as with all trains you use with the pass.