I have a year free apple TV, how do I get a verification code to enter it on my tv?

What is your model Apple TV? Identify your Apple TV model - Apple Support

If you have activated two-factor authentication for your Apple ID account, then you would receive any verification code on your secondary trusted device. But that is not related to the Apple TV+ subscription itself.

Two-factor authentication for Apple ID - Apple Support

To enjoy you free year of Apple TV+, you have to redeem the offer first, on your new Apple device, in the TV app. It does not depend on a redeem code, but instead on an identifier of your new device in combination with the registered Apple ID account for that device.