You need for the bus a reservation that can be done on the Website of rhb.ch price Was arround 5-10€

you may make a detour and catch the italian trains to Tirano (no reservation needed ) 

Im sorry i should have asked :

I'd like to take Bernina express bus from Tirono to Lugano but how can I select that on the app ?

As i said you can reserve on rhb.ch:)

https://www.rhb.ch/en/panoramic-trains/bernina-express

Just select Tirano - Lugano reservation for the bus cost in summer 16CHF and indicate at the end you have a Railpass :)

Thanks but you are not addressing my question. I appreciate you can book via the operator website BUT I'd like to book via the Eurail app. That's my question - how can you book via the app?

I'm sorry, no idea… I never use the app to make reservations because it charges extra (2 EUR per person and reservation). I believe the same is true for seewulf...

You cant reserve this bus via the App Thats why i suggested you how to do it (with the Eurail option) You dont have to use the page i suggested. You can try to get a seat on the Day of travel but as the bus is popular and just arround 50 seats its often sold out :) 

As said reservation for the bus in Summer cost 16CHf :)

And as @rvdborgt said iam not a friend of the App as they charge a Service fee for the bookings (I only use them if there is no other choice and this happend just once on the last 8years ) 

Kind Regards 

seewulf