Hey,
Do you want to book for 1 person or several people?
On the forum here you will find many reports of people having problems when they want to book multiple tickets in combination with a mobile pass.
If you have a mobile pass, and you want to book for several people, you have to email customer service.
You can also book via the B Europe website, for Thalys.
They charge 4 euros per train, regardless of number of travelers if I'm not mistaken.
Via Interrail you pay 2 euros booking costs per traveler, per train.
https://www.b-europe.com/EN/Booking/Pass#TravelWish here the mentioned page of b-Europe (SNCB Belgian Rail) to reserve Eurostar and Thalys services :)