RESOLUTION
Plesk for Linux/Unix:Plesk license number can be found at the Server -> License Management page in the Plesk administrator CP. Check the "Key number" field. Also it is shown in the title of the browser window when you are logged into Plesk CP as "admin" user.
If the Plesk CP is not accessible you can login to the server via SSH and know the license number with help of the following command:
# grep 'Key number' /etc/psa/psa.key
Key number: PLSK.00XXXXXX.0000 Plesk for Windows:
The license number can be retrieved using any of the following ways:
1. In Plesk control panel go to Server, License Management and see the license number there.
2. Use the command in the command prompt:
"%plesk_bin%\keymng.exe" --get-key-number 3. Open Registry editor (Go to Start, Run, type regedit, hit Enter).
Open the following key for editing:
HKEY_LOCAL_MACHINE\SOFTWARE\PLESK\PSA Config\PSA Key\Key
<value>KEY_NUMBER</value>