The pass is your ticket.

For some trains, like Thalys, their are mandatory reservations. Thalys reservations, you can get here:

https://www.b-europe.com/EN/Booking/Pass#TravelWish

For which you need a pass cover number:

Note, you’ll need to book until Brussel Zuid/Bruxelles Midi, and take another train to Central (only 4 minutes away), trains almost every minute on weekdays.

On the Thalys you’ll need to show your pass + reservation (usually a pdf), without reservation you cannot board.

Passholder seats are limited on Thalys, and quite expensive. You can minimise reservation cost by taking a TGV to Lille, and continuing your journey with reservation free regional trains to Brussels (slower, but a good option should the pass holder seats sell out).

And alternative via TER to Mabeuge, local train Charleroi, and then IC to Brussels exist too, without any reservation. But much longer travel time.

@BrendanDB thank you for your answer! I already tried on that website but it can’t find any train on the date I need (10 april). I really don’t know how to book it

Then the passholder seats for Thalys have been sold out.

Book a TGV from Paris-Nord to Lille here:

https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish

From there you can take a train to Brussels (change in Tournai or Courtrai/Kortrijk or Gent).

Edit: I see that there are replacement busses between Lille Flandres and Tournai, so taking the train from Lille to Courtrai/Kortrijk will be your best option.

It would look like this - connection available every hour :

The train LilleFlandres - Kortrijk and Kortrijk- Gent is actually the same, so you can stay in the train.

I can recommend you to visit my hometown, Gent, by the way ;)

Alternatively, if you want a faster option, buy an ordinary ticket Lille-Brussels with TGV (costs 19 EUR, cheaper than the pass holder reservation fare…, Leaves from Lille Europe), but this connection is not that frequent and has a big hole in the schedule in the afternoon, whereas the regional train has hourly connections.

You can buy that ticket here: https://www.b-europe.com/EN