I didn't have an Amsterdam loaner, but from September and I think the procedure of returning is the same.

Look into the loaner agreement/NDA. There is an address where you have to send the device (at your costs, I presume). I added a short note in the package requesting a short e-mail confirmation upon receipt, and got the confirmation.
Just make sure, you include everything you got. In my case it was device, battery, and charger. In your case, it might have been the complete retail-box.