Thalys can be booked in the same way as the Eurostar, please read more below. 

There is a limited number of pass holder seats on the Eurostar and on popular departures and during high season those sell out weeks, and sometimes months in advance.

The best place to see the availability of passholder seats and make reservations on the Eurostar is 

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

Press "later trains" and eventually you will find the next available connection. 

If you have a mobile pass you need to generate a Pass Cover Number in order to make the reservation at b-europe. You do that here in the PCN generator:

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653

Passholder seat reservations for Thalys can best be made through https://www.b-europe.com/EN/Kopen/Pass#TravelWish (€30 reservation per seat + €4 booking fee per booking), or alternatively through the eurail reservations portal (€32 reservation per seat).

OR IF wanting to save rupees and also willing to change more-with thus also chance for some visits on the way to towns you would never have dreamt about-its quite possible to do it for less or even without any extra to pay. App also has a feature of ´only RES free´.

Please mention time and date for the best advice and guidelines. When will you be travelling exactly? Train numbers like THA9328 will run (almost) daily.

Thalys is usually not good value for money, especially between Amsterdam and Brussels, with an extortionate reservation fee and limited value for money.

Better take the hourly, reservation free, intercity opening up places like Rotterdam, Breda, Antwerp, Mechelen,... for spontaneous visits. Especially Mechelen is remarkably nice, and very unknown. Perfectly doable in a couple of hours. 

Thalys is the most time efficiënt way to get from Brussels to Paris, although (slower, but much less reservation costly) alternatives exists over Lille or Charleroi/Maubeuge.