Hello BLiKing
You have to choose a connection from Lauterbrunnen to Jungfraujoch via Wengen, via Kleine Scheidegg.
The ticket does not include a seat reservation. If you want a seat reservation in addition, you have to pay for it extra, and get it from the operator’s website, not SBB.
In my opinion, it is best to book that ticket directly from the operator, the Jungfrau railways, either online or offline.
https://www.jungfrau.ch/en-gb/jungfraujoch-top-of-europe/buy-jungfraujoch-ticket/
Also, it is not advisable to purchase such expensive tickets in advance. What if you will face bad weather and you won’t see anything up there? Would you still want to go? Purchase the tickets the day before the excursion day at the train station office in Lauterbrunnen, when the weather forecast is reliable. There you will get everything: the ticket for the desired route, and the optional seat reservation.
Kind regards,
Roland

