Ok, here's the booking procedure from the ÖBB app/website.
While you can travel from €49,90 in the 2nd class, I recommend at least once the 1st class Panorama Car. You have 23 intermediary stops, on a gorgeous route, but you can split it if those 9:31h are too much. It can also be taken from other stations, as I might give it a first try on the Innsbruck to Graz section.
Anyway, back to the reservation. Select 1st class, click on "Seat reservation", then choose from "Select a coach" the Panorama car 1st class, and then the preferred seat if that's the case. Ticket + reservation, €82,90.
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