score:2
On voyages-sncf, the standard booking tool does not provide a field for your "Code Avantage". However, if you go for advanced search, you will find, in the section for the passenger details, a "Promotional Code" field appearing.
I know it is not straight forward.
Upvote:2
If you go to the voyages-sncf webpage, you'll see there's a field called Code avantage
. Just type your code in, and the associated discount should be applied.