Apple TV 3rd generation

Hi Jocee, do you mean a passcode for authenticating your AppleID? If that is the case, here are the Apple Support doc directions:

If you're asked to enter a verification code

If you have two-factor authentication turned on for your Apple ID, you might be asked to enter your six-digit verification code when you sign in to your Apple TV with your Apple ID. Get your verification code from a trusted device with iOS 9 or later or OS X El Capitan or later, or have it sent to your trusted phone number. Then enter the six-digit verification code on your Apple TV.