Hello Austin Wu, 

best done here: https://shop.oebbtickets.at/en/ticket?cref=oebb-allgemein - choose "one-way tickets ..." and your pass as discount. 

Reservation is required.

You'll get Business class.

Kind regards, Hektor.

Hello Austin 5

In fact trenItlaia has made it impossible to book extra seat-only online-or on their own machines, if you are in IT. have to visit counter. This due to rampant fraud in the past. OeBB is austrian railways and this is a kind of sneeky workaround. Any railway when you travel further has its own ways-many do not really want you to make RES all the time, but ESpana and FRance are the very worst for that. Good luck with it all.

There are many ways to directly RES and not via app-cheaper too. The usual posters here will follow and send a link to a long list for that per country.

Thank you

Looks like OeBB is a way to use it. Any down side?

No downside, you pay less than via eurail.com because ÖBB has no booking fee. 

Only some trains sometimes can't be booked. Should be a bug.

It is not possible to have a specific seat, but this is not possible either at eurail.com. Maybe italian ticket offices can do this. Also other ticket offices/call center of like SNCB/NMBS, NS or DB.

Frecciarossa cost 10 EUR and IC 3 EUR.