Q: Where to find agreement registration number?

Are you referring to the Applecare Plan registration number?

If you purchased it along with your device directly from Apple, they have registered it for you automatically. If you purchased it separately, you need the small booklet inside the box as that contains the number which must be registered within 364 days from date of purchase of a computer or within 90 days for an iDevice. You will not be covered by Applecare unless it has been registered and associated with a device.