Using an SNCF Coupon while booking on RE Site

I am trying to book a TGV voyage from Paris to the south; a route I have taken many times in the past. I have a coupon, sent to me by SNCF, of about $100.00 value, due to a late train on a previous trip. It says on the coupon that it cannot be used "after the fact". I previously wrote to RE and was told the coupon was valid, but no one seems to know how I actually deploy this coupon. Any thoughts?
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