Hi Marit, for what date are you trying to make the reservation?

For tomorrow 17/07 at 8h43

You can reserve directly with b-Europe note they charge a booking fee of 4€ https://www.b-europe.com/EN/Booking/Pass#TravelWish and seats for Railpass user are limited :/

Alternatively, go to a Belgian railways ticket office, where there should not be any extra fees for a reservation. Do note however that for tomorrow, only the 10:13 train from Brussels seems to have Interrail seats left (25 EUR + 4 EUR per order).

Alternatively, you can take an IC to Lille (reservation-free) and take a TGV Lille - Paris. Belgian railways can also book this TGV (10 or 20 EUR) at a ticket office at the bigger stations.

As seewulf mentions, there are only limited seats available for Pass holders. Rvdborgt is right. You can either take the 10.13 one or go for the alternative that is mentioned. Have a good trip! Cheers, 

At the weekend there is also a regional train between Lille and Paris, so included in your interrail pass