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

with a pass cover number from cover of paper pass or using this link to get cover number for mobile pass

Be aware, Thalys have hefty reservation fees for passholders and small quotas so you may find availability patchy, using other trains/routes may be a better option.

In addition, Thalys requires a pass that is valid for the entire length of the journey. If your pass is not valid in Germany, then you can't use Thalys to cross the border.

So-either-depending on where you precisily live-go by local train just over border-but also bcse of very high surcharge on TLs-and often even no seats for passholders, it may well turn out to be  a very bad buy as normal tickets may work out cheaper. I would advice to adjust travel plans.

Mostly the Thalys trains are not showing up for booking as it may be already booked. Are there any other trains from Paris to Amsterdam and Vice Versa. When checked in https://www.sncf-connect.com/ other trains are showing as “Reservations can not be made”.

On the SNCF website you can't book pass reservations anymore. SNCF removed that possibility (and Thalys could never be booked there).

The best website to check for Thalys availability is:

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

It only shows trains with availability. Keep clicking "Later trains” until you find something. Availability is better on Paris - Antwerp and again on Paris - Brussels, so also check those (use the hourly reservation-free intercity to travel further). Another alternative is to use the TGV from Paris to Lille:

https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish

From Lille, continue with intercity trains to Kortrijk - Antwerp - Amsterdam.