Hello Kothuefen52
The seat reservations are to be made with the train operator, not with SBB. You can find this option on the Glacier Express website:
https://shop.glacierexpress.ch/gex_en/specials/daytrips/glacier-express
A seat reservation can only be made 92 days before the day of travel. When this deadline has been reached, you can certainly just reserve the seat without purchasing another ticket. Simply proceed as you would want to purchase a ticket, choose the route, choose the connection, choose a seat, and then click Next.
On the next page, select ‘Reservation only’ from the Reduction dropdown field; the ticket cost is automatically deducted:
Then, you are all set.
You can do a dummy booking for any train in the next days to see how the reservation process works.
Please kindly keep in mind that Saver tickets need your physical passport or ID card (no photos, no paper copies) upon ticket check on the train.
Kind regards,
Roland