Not sure if this applies to any other 3rd party retailers, but Amazon customer support will be able to give you your early access code before you actually receive your physical edition.
Just sign in to Amazon and go to customer support. At some point, there will be an option to "Contact Us". From there, select the "chat" option and ask them for the early access code located in the annotation section on your order. Only they can see this section (as far as I can tell). Enter that code in the "Redeem Code" section of your Elder Scrolls Online account. I hope this helps some people!