$serialnumber hidden field not generated in the form

I want to save the value of the $serialnumber together with the other fields, via the form finisher form_to_database, but it seems that the field: serialnumber is not generated in the form, even if the Enable additional serialnumber field is activeted?

Can you confirm that, or am I wrong. How do I save the serialnumber in the database together with the form fields?

Martin Kristensen