In order for the product code from the NKT reference book to be automatically inserted into electronic invoices, the corresponding setting must be enabled in the configuration — simply having the codes in the NKT database is not enough if the filling function itself is not activated.
How to do this
Go to the General Settings section, then open Electronic Invoice Settings. There, switch to the "ESF Filling Settings" tab.
On this tab, find the flag "Fill in the product code in documents based on NKT data" and enable it.
After enabling the flag, the system will start automatically inserting the code from the NKT reference book into ESF documents, provided that the product is found in the NKT database and a corresponding item is matched to the code.
If codes still do not appear, check that the setting has been saved and that you are working with a new ESF document created after enabling the flag — documents that have already been generated may require the codes to be filled in again manually or by refilling the table section.