eboy909
Seat reservations are usually managed by the operator company. You could also buy the ticket from Zentralbahn together with that reservation, but this would only make sense if not using other operators / means of transportation that day.
Note that on that particular train, only a few coaches will have reserved seats, there are no reserved seats in the majority of coaches. I took that train between Meiringen and Luzern one month ago, and I had that 2nd class coach I’ve boarded (probably one without reserved seats, didn’t really check, as I never reserve seats) almost on my own.