Hi there, thank you for asking. You do not need the box for registering your tablet.
In one hand, you can find the serial number of your tablet on the back side of this one.
On the other hand, in order to register your software you must have the Wacom Driver installed and with that an application called Wacom Desktop Center installs, from there you will be able to register your tablet as long as you have it connected while you are taking such step. If you need any help registering your tablet, you can reach out to 1-855-MY-WACOM. Hope that helps. Best