Hello Nuennijeal58
Yes, you can, but, if you want to reserve a seat from Zurich to Salzburg, which is recommended, you must do it separately on oebb.at or in the ÖBB app. On the SBB app you can only reserve a seat from Buchs with the ticket.
In this case I would also buy the ticket Buchs - Salzburg from ÖBB because it is an internal trip in Austria. Please note that ÖBB-tickets are no more refundable if you get them as PDF-files in advance.
Kind regards
Danhiel