how do I get the 1 year free AppleTV service when buying a new iPad

You activate it in November when its made available.

One can assume there will be the option to verify the purchase when you start the subscription.

Apple TV+ launches November 1, featuring originals from the world’s greatest storytellers - Apple

Customers who purchase any new iPhone, iPad, Apple TV, Mac or iPod touch starting September 10 can enjoy one year of Apple TV+ for free. Beginning November 1, customers can initiate the one-year free offer in the Apple TV app on the device running the latest software. Customers have three months after device activation to claim the offer, or if the device was purchased and activated before the launch of Apple TV+, they will have three months starting November 1. The subscription will automatically renew at $4.99 per month after one year. Customers can cancel at any time in Settings at least one day before each renewal date. Customers who cancel during the offer period will forfeit the remainder of their offer. This limited time offer applies to both new and refurbished models, including devices from the iPhone Upgrade Program, is not restricted to any specific sales channel (e.g., Apple Store, resellers) and will be available in all countries where Apple TV+ will launch. Up to six family members can share one Apple TV+ subscription and watch using their own Apple ID and password. Only one one-year offer is available per family, regardless of the number of devices purchased.