You have to first log-in to your FAA account at FAADroneZone:
Click "I agree":
Then find and select the blue box that says "Launch Drone Owners and Pilots Dashboard":
Select "Manage Device Inventory:
Select "Add Device":
Fill in the form, then click "ADD Device" in the box in the lower right:
IF you are not a Part 107 pilot and are registering a module that can be swapped between multiple drones, enter the word "shared" for the "UAS Manufacturer" and "UAS Model" fields. The "Serial Number" is the serial number of your module.