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 on the Eurostar is 

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

There will be no pass holder seats available at the station if there are no seats avaliable through sites like, b-europe.

What route, date and time do you want to make a reservation for?