If you've signed up for DroneDesk and have a license key, then you'll have received an email with the link to sign up. If you've misplaced the link, then you can come to the DroneDesk homepage, dronedesk.o, click on sign in, and then click on I have a license key where you can enter your key. Tab out of the field. It'll be validated and then enter your business details, your name, set your password, and click sign up.
You'll get an email confirmation, but you should be able to log straight in with the email address and password that you set.