Hello!

On the 16th I would like to take the intercity bus 835  from Villach to Venezia. The interrail app tells me that I need to book a reservation, but I can’t do so trough the interrail system because it wouldn’t get to me fast enough in the mail. 
I can find the bus on the official öbb website but it does not allow me to just book a seat reservation. 
 

Can anybody help me with this? Do I have to make the reservation in person? Will I be able to in Villach? Or can I just not make a reservation?

Thank you in advance!