Last November I bought an Xbox One S and a bunch of games that came with a couple of free trial card for Xbox Live Gold.
Since they seems to be only working if you never were a Gold Member in the past, I am unable to use them.
My guess is that those cards are used to bring a friend to Xbox Live Gold in that case, but since my friends and I are more of a PC/PlayStation/Nintendo gamers than Xbox, I have no one to give those codes to.
So I will simply type them below, feel free to take them.
14 days trial #1: F6DYN-RCXXQ-9J7QM-376Q9-TF3H4 (came with the console)
14 days trial #2: 74CJB-6GFXC-K6FB4-RYYCF-3BY34 (came with Forza 6)
2 days trial: RMFXH-NYGDW-HQK9M-RWTD6-23674 (came with Halo: The Master Bagel Collection)