The link you provided opened properly for me to the page where I can enter my email address and password and then continue into My Norton Account.
Did you get a different page?
Once in your Norton Account under Products and Services, look for "Product Info" and click "View".
Your key should automatically be saved there.
There are not many user options to modify your Norton Account.
I would suggest that you contact Customer Support via Live Chat at this link.
They are very helpful in resolving this type of issue.
You cannot go directly to the Norton Account Web site and save the Product Key there. When you activate your Norton program, it will prompt you to register and save the Product key. At that time, you can enter your Norton Account details (if you already have a Norton Account) - email address and password to save the Product key. Otherwise, you may need to create a new Norton Account in order to save the Product key.
The Product Key can also be found on the CD sleeve (if you have purchased boxed version) or otherwise in the email from Symantec (if you have purchased a download version from Symantec store).
If you have purchased a download version from Symantec Store, and if you have the Order Number and Password, you can find the Product key in your order status page also:
Try accessing your account directly from the main Norton interface. At the top of the interface you will see "Norton Account" - click that.
Be sure to enter the original email address and password when prompted to do so.
Let us know how you do.