Sure, use https://www.nightjet.com/en/#/home or https://shop.oebbtickets.at/en/ticket

Select the correct number of people and add Interrail as a discount for you only.

Let us know if it doesn’t work. :)

Here is a general guide on seat reservations (Interrail is rarely the best place to book) : https://www.seat61.com/interrail-and-eurail-reservations.htm

Thanks for your quick respond! 

I tried to book tickets with the second website, but I didn’t had to fill in a reference number. 
Did I do something wrong or do I have to show my Interrail pass in the train? 

Thanks for the general guide! Very handy! 

Yes don't worry, simply show the pass onboard if they ask.