Updating for completeness. No longer possible via Twitter.
The Thalys TheCard website has a chat function. I just had success asking this using this chat. According to the person I chatted with they are allowed to link 1 trip a year to your card manually.
Originally Posted by
bruce80
Points will only be credited after the conductor scans the ticket or card? Good to know (although I'm not a Thalys member), this is different for DB where points will also post if the journey isn't taken (unless the ticket gets refunded).
The miles for my outbound trip immediately appeared on my account. The inbound trip has not yet posted.