This is the current AMD Promotion concerning AMD GPU cards: Radeon Raise the Game Bundle - COMPONENT AND SYSTEM INTEGRATOR (SI)
The Retailer or Vendor where you purchased the RX5700 needs to give you a Coupon Code to redeem at AMD REWARDS website. They also need to be participating in the AMD Promotion. If they are participating they should have sent you an email or some other notice with the Coupon code for you to redeem. If they didn't you need to go back to the Retailer or Vendor and ask for the Coupon code. AMD doesn't give out Coupon codes only the participating Retailer or Vendor.
Here are the Terms & Conditions for the current RX5700 AMD Promotion:
When you redeem the coupon code at AMD Rewards, it is imperative that you have the RX5700 GPU card installed in the computer. AMD REWARDS has you download a small Verifying program to make sure you have a qualified AMD GPU card installed.
But the most important action you need to take is go back to the Retailer or Vendor and ask them to give you your Coupon Code to be redeemed. Otherwise you are up the creek without a paddle (-: