Usually, when you first turn on a new Kindle or one that has been reset, you’ll see a setup screen. You’ll then be prompted to select your language and connect to a Wi-Fi network. After connecting to Wi-Fi, you’ll see a screen asking you to register your Kindle.
On this screen, you should see an option that says something like “I have a registration code” or “Enter a registration code.” Tap on this option. You’ll then be prompted to enter your registration code. This is where you input the code.
After entering the code, you’ll likely be asked to sign in with your Amazon account or create a new one if you don’t have one. Your Amazon account password comes into play only after the registration code has been entered and accepted.
You can also register a Kindle and enter a code by going to amazon.com/code and then entering the 6-digit verification code you received.