Please provide me following details-
1. Your admin user name, password and site URL by using this form- http://wedevs.com/provide-login-details/
2. The form name which you are using.
3. Page name which contains that form.
4. The file name which you tried to implement.
I assigned this issue to a developer who is now in vacation. He will be available from friday. After his coming back he will check this issue. Please keep patience. I am sorry for your inconvenience and after his coming back i will inform you the update.
I am sorry for the inconvenience. Our developer tested what you wanted to do. We do not have any built in hook right now to add this extra custom field. Its need to be customized. So it will take many time to do this custom field. I will suggest you to hire a developer to make this custom field for you.