score:6
As drat pointed out in his answer, it is actually possible to specify your Eurostar Frequent Traveller number when booking through SNCB, but you must do it at the start! (Most schemes prompt for it towards the end, which caught me out)
If you have booked your ticket without your Eurostar Frequent Traveller number, there are two options available. One is to wait until a few days after you travel, then put in a retrospective claim. The second, which I've just discovered, is go to the Eurostar website and do a "Contact Us" for "Already Have A Booking" then "Frequent Traveller". Let them know the booking reference (for an SNCB reservation you need the PNR not the DNR), along with your Eurostar Frequent Traveller membership number, and they'll add it to the booking for you!
Upvote:6
Contrary to your statement, you can add the number during the booking process. When you book from SNCB you can tick Loyalty cards.
In the second step you can then add the Eurostar Frequent Traveller number:
I know this doesn't help you anymore since you already booked it, but this is how you can add your frequent traveller number.