This one is super simple.
Log in to your Amazon account.
Go to https://www.amazon.com/b?ie=UTF8&node=19700960011 to go to the promotion page. You should see a "Your offer is now activated" page. That’s it! Now all you need to do is watch a video included on Amazon Prime and the $5 credit should appear in 3-5 days. My recommendations are Marvelous Mrs. Maisel, The Americans, or some classic Spongebob 😎
If you're not a Prime member, you can sign up for a free trial, immediately cancel it so it doesn't auto renew, and then I think should be able to do the above.