Installing already PS CC, now I'm asked to fill in Serial No.
Please advise how to get it. Could someone send me th URL.
The screen you posted would list any purchased software products if you had registered any previously (perpetual or purchased software uses a serial number).
Since Creative Cloud uses your AdobeID to associate your subscription to your software, there is no need to enter a serial number for software you subscribe to in the Adobe Creative Cloud.
If you are being prompted to enter your serial number when opening your actively subscribed Creative Cloud application then please see http://helpx.adobe.com/creative-cloud/kb/ccm-prompt-serial-number.html for additional information.