Hello Bar16
Seat reservations are very uncommon in Switzerland and not mandatory. Here, the train is considered mass transit; Swiss people simply get on the next possible train without loosing a thought on seat reservations - our trains have free seating.
Consequently, seat reservations are possible for EC / IC / TGV / RJX / ICE trains only, not for IR trains.
You may wish to read our blog on the topic:
https://community.sbb.ch/blog/23887-demystifying-seat-reservations-on-swiss-trains-a-comprehensive-guide
For your journey, a seat reservation is defintely not needed.
Kind regards,
Roland