In Switzerland, train tickets do not have a seat assignment as tickets (except of type Supersaver) entitle to use any train connection regardless the time of the day.
Although we have a so-called nationwide integrated ticketing system, meaning tickets can be bought with any Swiss public transport company for any connection in Switzerland, and at the same fare, one who wants to reserve a seat in addition, has to do so with the company operating the line, in your case MOB:
So, to conclude: buy a ticket on sbb.ch, or with the ticket clerk or ticket machine on-site. And reserve your seat in advance. You do not have to hold a valid ticket in order to reserve a seat, however.