Registration of a product to your account occurs automatically when you add a Sonos device to your system.  No further action is needed.

Thank you John. the problem is I had this equipment in my former house. It is not connected anymore so I cannot extract that info

If you previously had the device connected to the Sonos app under your account then the device will show under your Sonos account online.

However, if the device has since been added to another Sonos account, they will now be the registered owner and it will no longer show under your online Sonos account.