AutoFill has been an excellent and convenient technology in the Apple ecosystem for years now, first on the Mac, then on iOS. Now, your Apple TV can take advantage of AutoFill on your iPhone or iPad for login credentials, whether you're entering your Apple ID or your Netflix login information.
You'll need iOS 12 or later installed on your iPhone or iPad, while your Apple TV will need to be running tvOS 12 or later.
How to use AutoFill on Apple TV
Autofill will become an option when you encounter a username and/or password field on your Apple TV, such as when you sign in with your Apple ID.
Navigate to a username or password field on your Apple TV. This could be your Apple/iTunes ID, your username and password for apps like Netflix, Hulu, and HBO NOW.
Tap the Password AutoFill notification that appears on your iPhone or iPad.
Enter the authentication code that appears on your Apple TV into your iPhone if you are asked to do so. Tap the username and password option you'd like to use. Note that since you're using iOS 12, third-party password managers can offer their suggestions here, too if you've enabled that option.
Authenticate with Face ID, Touch ID, or your iOS device's passcode.
Some of these steps, such as entering the authentication code or the two-factor authentication code, are highly dependent on what type of account you're entering. For instance, if you're just entering your Netflix login information, you'll likely not need to an authentication code to connect your iPhone and Apple TV.