It says in the FB Plat that you are entitled to 15% Discount on SIXT and an upgrade (if available). AF gives you a code 91......

Tested it this weekend and cancelled my last reservation with SIXT made a new one entering my FB number. No input ask for a code and get the same rate as before.
Send a question to AF .... No reply as usual. Send a question to SIXT they said Discount was applied automatically. Checked it by making a reservation anonymous.... Still the same rate.
Called SIXT today and a very friendly customer service agent told me that I have to make a copy of my FB Plat card and it will be send to the system and then the discount will be applied. Wait and see