The SBB website mentions this:
You can reserve your seats on Swiss trains at SBB.ch, SBB Mobile, via SBB Contact Center 0848 44 66 88 (CHF 0.08/min.) or direct at the ticket counter. It costs CHF 5 per seat to make a reservation.
However, I do not seem to be able to find where that is possible…
Adding a link to where you can reserve to that text would have been the minimum…
To the OP: You could try to reserver you seat at the ticket office in Innsbruck (or any other major station). Otherwise, I would in your place just board the train in Zurich. Reservation is only compulsory between Chiasso and Como, and that is only a few minutes…