Yes you need serial number to tied up the demo license to that respective device.
Here is how to find serial number :
serial number can be identified in the following ways:
- GUI: Monitor/Reporting > System Status
- CLI: use the command: version
here is the instruction to get License.
https://www.cisco.com/c/en/us/support/docs/security/email-security-virtual-appliance/118301-technote-esa-00.html
If you still have issue, suggest to contact local SE / or contact cisco license team to help you.
Pease contact our Global Licensing Operations team:
https://tools.cisco.com/SWIFT/LicensingUI/Home