To get the Device SN press the Forward key on your device when the Settings message is displayed. Depending on the device model, you may need to press the Forward key more than once until the SN appears.
See your device's user manual (The Settings mode chapter) for further details

Insert the whole Serial Number with hyphens.
You cannot associate more than one account to the same Device SN.

This procedure is not applicable to older atLEAF+ devices, please see this page