Following the help given in this Travel.SE answer, I now know the trick for specifying my Eurostar or SNCF frequent traveller number when booking a ticket through the SNCB website. Now, I find myself with the same problem with Loco2
The Loco2 homepage is very good at asking you about any railcards you might have, and offering discounts on them, so it's good in that regard:
Despite that, I can't seem to spot anywhere during the train selection or booking process where I can add a Eurostar Frequent Traveller or SNCF Voyager number. Is it possible to do so, and if so, what's the trick?