Hello Paolo,

Yes, you could reserve seats without activating your pass. Please also note you don’t need to book trains in advance in many countries, unless you want to reserve a seat, e.g. Germany. 

If you have EUrail pass, you can directly reserve seat with this link: https://www.eurail.com/en/book-reservations#/

For the Interrail pass holders, I think you can also easily find the corresponding link with Google.

@Ruchuan @Paolo Bianco 

The advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail/Eurail website. Far from all trains can be booked there and there is an extra fee of 2 EUR per ticket. You can look at the guide in the link:

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

If you, after having looked at the guide, have questions about how to make specific reservation, please give your travel details (departure date, time and route) preferably in a new post, and you will get advice.