Extrafields: constant not stored in database

Hi, I have defined new extra field with a value constant in the extrafields for the invoice, it shows properly in the screen but later is not saved in the database. Is there anything specific required for storing the information?

The situation is the following: the government provides an specific text for invoice range authorization that needs to be printed and stored against each of the invoices done in the system. Once the range ends or the expiration date, passes a new authorization is requested thus a new text to store in each of the new invoices.

Everything is being done at the GUI layer of Dolibarr currently.

To bypass the printing situation,I included as a patch the text constant, but would prefer to be automatic so the user can handle it without any programming modifications