Autodesk web services restricted by login (name/password) use a "single sign-on" system based on "User ID".
This common login name can be then used to download add-ons, for Autodesk web-cloud services, for the Subscription Center (if you have an active subscription), for Autodesk Labs, for the Students portal, etc.
If you don't have your personal "User ID" yet (e.g. from your subscription contract), create a new one. On the page https://register.autodesk.com choose "Create User ID now", enter your personal data, choose your "User ID" and password (must contain 8 to 12 characters, at least one letter and one digit). Submit the registration. After you click the confirmation e-mail message, your User ID becomes active.
The registration is free and it is not bound to any purchase of Autodesk products.