- You buy an installation disk that you then use to boot the device,
- you run the custom image wizard [to confirm the hardware it has found is correct and to exclude unwanted software modules] and
- you put in your product key [which is always a volume licence key supplied as part of a specialist type of MSDN subscription].
- Then you develop the customised functions needed for the tasks that have to be done on the device and
- then you make a customised production image.
So you need to get the customised production image from whoever did the development for the device.
They will already have embedded the product key into the image because it is, by definition, an image that gets deployed without any further amendments.
If you are replacing the hard drive like-for-like then perhaps you could make an image from the old HDD to put on the new HDD. But you would need to study the support documents available on the Windows embedded website to find out if that is feasible as
well as how to do it. They also have a forum within MSDN & there is a link to that there as well.