Why do you think this train is fully booked? You can book reservations via tickets.oebb.at with the "Seat only” option.

If that doesn't work, then please specify your travel date.

No reservation needed for that route and also train is usually not very crowded (at least across the border, inside the countries there might be some people on the train).

thank you @rvdborgt if i book of the webiste you said that will that still work with my interrail ticket if i just get a seat reservation ? 

Sure. You can book a reservation anywhere you like. Until now, I've managed to avoid booking via Interrail and thus avoided their fees and overpriced optional reservations.