Log in with your Prime-enabled account or sign up for Prime. Click Continue  to begin the account-linking process. Click Go to Activision  to be redirected to Activision's account-linking page. Log in with your Activision account and authorize Prime Gaming to fulfill content.

One must log into their Amazon account with the Prime subscription to get the redemption code for free rewards. Copy the redeem code. Tap the //callofduty/redemption  and paste the redeem code in the text box.