Hi, 

if reservations are open you can reserve it for less money directly via ÖBB (often they open late on this route due to timetable changes due to works on the line).

Go to oebb.at and look for this route and select the seat only option. It cost 3 EUR per seat (Instead of 6+2 EUR of Interrail/Eurail). Reservation is optional.

If I buy the seat reservation on that website, how do I add it to the eurail pass?
I'm sorry, I'm new, I've never used the pass before.

Reservation via ÖBB is even free of charge in 1st class. Reservation is open on tickets.oebb.at.

You don't. Reservations are completely separate from the pass.

You still have to add the journey to your pass but you only need to do that before you board the train.

but can I add the trip now to the app?

Sure you can. But there's no real need such a long time in advance.