You actually do not need to be in posession of the STP in order to make a reservation for the Glacier Express. Reservations and tickets live in different universes.
In order to travel on the Glacier Express you need:
a) a Ticket, or something that stands in lieu of a ticket, like a Swiss Travel Pass.
b) a reservation.
So go to www.rhb.ch and reserve your seat, and indicate that you will have the Swiss Travel Pass, or just select “Reservation only”, the effect is the same…