TYPO3 Form Finisher: form_serialnumber and savetodatabase

Question

I'm using @armin 's form_serialnumber extension. It works fine: I can receive the serial number in the email sent by the EmailToSender finisher.
I can't find how to register the serial field with the SaveToDatabase finisher. Any suggestions? I have tried various methods including:

    Serialnumber.serialnumber:
      mapOnDatabaseColumn: serial

or

    serialnumber:
      mapOnDatabaseColumn: serial

Thank you
Roberto