You can try to book with b-europe but you need a paper pass cover number. You can request it via costumer service buty they are busy. 

You can also try to book it via phone with Belgian Railways SNCB or at International Ticket Offices of NS, SNCB/NMBS or SNCF. 

Okay thank you. Another question - do you know if I can book tickets directly from DB with the “Only seats” option?

On the 17th of June there are no passholder seats available before 13.15 from Amsterdam to Paris on Thaly. The passholder seat quota is sold out on the earlier trains.

Yes, you can book with "Seat only" at DB for domestic German trains and some international trains.

Okay great, thank you!! Just to make sure, I can buy the reservation from DB with “Seat Only” and then on the train have my Mobile Interrail Global Pass as my ticket?

Yes. Your mobile pass is always your ticket. You add the journey to your trip and just before boarding the train you add your journey to your pass and create your ticket  (a QR code).

Again: seat ONLY for optional REServ will not be asked for-as long as you sit in right train/seat/day. Db can ONLY do S-O RES for its own trains. Not for Thalys, in FR-plus that the rotuing for this trip seems rather strange to me-and also full of superhigh surcharges (ANY hi-speed in/out of FR over border)