Depends what and where you want to book.

For optional reservations in Germany, Austria,... it's the same process (and price) whether you've got a ticket or a pass.

Often for special passholder fares you can add Interrail as a discount (nightjet.com) or as a rail pass (Rail Europe).

There are so many companies around Europe that it's hard to have a similar process. We're already grateful that Interrail exists 52 years later!

Have a look at this guide and feel free to ask more questions afterwards: https://www.seat61.com/interrail-and-eurail-reservations.htm

Thanks for your comment. It's really that unclear. I want to go to Spain and am referred via your site to:
https://www.raileurope.com/en
When I click on that and try to book something. I can't indicate anywhere that I have an Interrail pass.
How should I do this then?

I cannot book anything for my trip from the Netherlands to Spain on the desired date. How is this flexible and easy?

Let’s say it from the start: Spain is the worst country for Interrail. RENFE doesn’t want to deal with passholders.

On Rail Europe (desktop version) you can click on add rail pass. Which trains exactly are you trying to book? We’ll be able to advise better. :)

For example the Lyon - Barcelona AVE (run by RENFE) is unavailable online: you must go on the day on the platform and speak to staff. Yes for real! Fortunately there are alternatives.