Very much like about anyone used to do in days of yore-even at this awful service stops of your societe Nationale.: show ticket, tell (or write down-a thin that young people seem to have forgotten, if language is an issue) what you want and pay the price. A REServ for just a seat is usually aorund 5-6€, but for those pesky INTern TGV over the border it can be well over 30-courtesy of again SNCF. THese are usually also the trains that are labelled as need to have paper proof.

Just say you need a reservation.

For which train is it? (origin, destination, date, departure time)? There may be other possibilities.