The link takes you to a login page, part of which states 'To access this page you are required to sign in with a Windows Live™ ID. This will help secure your information and protect your privacy.'
If you have a Windows Live ID then use it to login. If you don't have one then click the "Sign up now" button and follow the simple procedure to get one. Once you have an ID and password make a note of it and keep it safe for future use.
Now you can login and fill in a form whereupon you will receive an email with a link to get the Product Key.