Hi Clara, it should send you to a page showing how to make seat reservations per country. Here you can find all the alternative ways to book. For the Thalys in particular you can have a look here. Cheers, 

Which Route? For the Amsterdam- Brussels section you can use a slightly slower reservation free Intercity If you wanna go to paris forget about that and have a look here (4€ Service fee will be add at the final purchaseing step ) https://www.b-europe.com/en/booking/pass#TravelWish