I'm not sure if this is correct for any other country, but in Australia we had to redeem the bonus code via the MySamsung app. I bought my new Gear VR with Controller from a second-hand site from a guy who had an S8 pre-order and it did not have a code in the box. I contacted him via SMS about it and he used his S8 to download the MySamsung app and redeem the code, he sent me the code and I used it successfully on my S7 Edge.
Long story short, Maybe you can get the code that's meant for your S8 on the MySamsung app?