When you go to the Samsung store and login, register your new phone. You will then be advised you qualified for the gift card code which you will receive in 6-8 weeks. I wasn't happy about this either
The easiest way to do this is by registering in the Shop Samsung app. The registration will be in your inbox.
They say 6-8 weeks, but it won't take that long. I assume they do need to space all the gift cards out so they don't have thousands of people buying the same accessories at the same time.