Register it with who? If you have set up your iPad and you created an Apple ID and iCloud ID., your device is registered. Your receipt will serve as confirmation of the date you purchased your device for warranty purposes. And if you purchased AppleCare+ (highly recommended - at least by me), Apple will send you paperwork with the coverage details.
If you’re trying to register an Apple product, chances are it’s already registered.
Most Apple products can be registered when you turn them on for the first time and begin the setup process.
Service and support for your Apple product are linked to its serial number, so it’s OK if you don’t register it.
If you'd like to check the service and support coverage for your Apple mobile device or computer.