Hello MAYBL95
Seat reservations are to be made directly with the train operator, in your case SOB, given the connection offers seat reservations at all as not all trains accept reservations. Seat reservations are not common in our country.
Please read here:
https://unterwegs.sob.ch/en/stories/reservations
Travelling by train for this route only, while you go will be going by car for the rest of the trip? You must spend a fortune on travel costs.
Kind regards,
Roland