You can always check via the site of b-rail-follow steps for passholders.

IF you book via that site you need pass-cvr nr and pay 4€ extra. (via app of this you pay always 2€ extra fee)

Since you book on a blue tuesday, off high season, I wouldn’t worry too much. You’ll definitely find seats to Brussels or Paris. Ordinary prices seem to be quite cheap, which means there’s still plenty of space on almost all the trains that day.

The best place to see the availability of passholder seats 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