The best way to make reservations for French trains is to call SNCF. No need of Pass Cover Number. 

https://www.sncf.com/en/customer-service/contact-us/telephone

 Press #85 for English, no booking fees, reservations are sent via e-mail

The advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail/Eurail website. You can look at the guide in the link:

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

If you, after having looked at the guide, have questions about how to make specific reservation, please give your travel details (departure date, time and route) preferably in a new post, and you will get advice. 

The easiest way to check Thalys (and Eurostar) pass holder seat availability is via b-europe (Belgian railways). It will only show trains with available seats. Keep clicking on "Earlier trains” and "Later trains” to find the last or first available seats before or after the date/time you requested.

To book there, you'll need a pass cover number though, and there's a 4€ booking fee per order.

You'll see that there's no availability Amsterdam - Paris or Antwerp - Paris on 17 July. There is still availability from Brussels though. You can use the hourly intercity Amsterdam - Brussels to connect.

But it seems you want to go FROM the ville lumiëre to Ams (the city that does not want any tourists anymore)? THEN use domestic TGV to LILLE-walk over to other station in 10 mins along price-crowned architectural monstrosity and find train to Antwerpen-admire this railway cathedral-go to lowest level-the BRU-AMS trains yellow/blue run hourly onward-the last 2 are locals with even possible to REServe. It will take some time longer but will get you there in the end-with afair chance to admire more on the way and for lower price too! And hey-I bet youre not the only one to do it that way-so make more friends on the way too!