Eurail overnight train reservation Prague to Vienna?

I am planning to take the overnight train from Prague to Vienna and another from Vienna to Venice. I already have a global Eurail pass. I cant figure out how to reserve my sleeping arrangements on the train without buying a full ticket. The pass covers both journeys, but I have no idea how to reserve. Id like to do this in advance since waiting till that day worries me that either there will be no seats or no seats together (there are 2 of us going). Is there any way to book now?