Hi,
When you are trying to extend the standard tables and structures we have to AET or EEWB option to append custom fields.
You have just added field in structure alone so you are able to use the field in web ui but when you save it, there should be a mapping field and logic to store the values in database table.
EEWB - Adding Table attribute - CRM - SCN Wiki
Regards,
Dharmakasi.