Thalys Reservation with Railpass

Here is the page from Thalys. https://www.thalys.com/be/en/offers-and-fares/interrail It 55 euro for a 2nd class reservation. If you just bought a ticket now, the price is 79 euro 2nd class
I do not know if you are required to make a 1st class res with your pass, if so, the res is 76 euro. You can also travel slower on local trains without the exorbitant res fee Thalys charges.

Thalys is one of the worst train systems in Europe for dinging railpass users with a supplemental fee. Sam has provided the details. Most times, you can buy an advance regular ticket for less than the railpass supplemental fee. Unfortunately, you are traveling in a few days and the discounted tickets most likely will be gone. But, you can try. Secondly, Thalys limits their seats to passholders. If those limited seats are gone on the train you want, you will either have to pay full fare or wait until railpass seats are available on a later train.

Your slower, non-Thalys options route through either Rotterdam or Roosendaal. Depending on the time, you may also need to change trains in Antwerp or Mechelen. None of these trains require reservations.

According to the timetables on the German Rail site, you can leave Amsterdam Centraal at 11:30, connect in Roosendaal and Antwerpen Centraal, and arrive at Bruxelles-Midi at 15:27. Or you can leave Amsterdam Centraal at 12:30, connect at Rotterdam Centraal and Antwerpen Centraal, and arrive at Bruxelles-Midi at 16:27.