RESOLUTION
Plesk Expand license number can be found at the Server->License Management page in the Expand CP, see the "License key ID" field. Also it is shown in the title of the browser window when you are logged into Expand CP under administrator account.If Expand CP is not accessible you can login to the Expand server via SSH and use the following command to obtain the license number:
# grep 'EXP\.' /usr/local/expand/etc/expand.key
<value>EXP.00524129.0001</value>