Fantastic, thank you! I was so close to the final step and this defeated me. I have now managed to book my SNCF reservation.
Next question: how can I do the same thing for an SNCF TGV from Luxembourg to Paris?
Unfortunately, https://travel.b-europe.com/eurail-ge/en/booking-tgv#TravelWish does not list Luxembourg as a valid station.
save money too: as LUX might be seen as INternat.=much higher extra. Board at 1st stop IN the FRancia, Thionville or perhaps Metz and get there before on very frequent local train.
INside LUX all transport is free since some 2 yrs. incl. buses/trams
Next question: how can I do the same thing for an SNCF TGV from Luxembourg to Paris?
Unfortunately, https://travel.b-europe.com/eurail-ge/en/booking-tgv#TravelWish does not list Luxembourg as a valid station.
The only way to book this TGV online is via Interrail (2€ booking fee per person and train).
You can also call SNCF. Press #85 for English, no booking fees, reservations are sent via email.