I have been using the auto generated facebook app-id for some time in my website. I had no knowledge of the App registration page back then. Is there any way I can still register my website in facebook using the app-id that I currently use?
As you get the app-id already, there is already an fb app registered and you don't need to register it again.
to link up your website w/ the app-id, simply go to fb dev app (https://developers.facebook.com/apps), set the site URL of the app to match w/ your website