jak21n You can book your seats here:
https://www.gpx.swiss/en/
If you have a first class ticket, you can choose normal seats or the Prestige class.
The first question is:
Do you already own a valid transport ticket?
Answer “yes”
You will see this indication:
To reserve a seat only, select “Only Reservation” under the “discount” option.
Select “Book now” to make the reservation.
It is not necessary to connect your Eurail pass with the reservation. Simply show both (Eurail pass and reservation) to the staff.