You can use the 123FormBuilder – iContact integration to associate form fields with custom fields built in your iContact account. In the Settings → 3rd Party Apps section of your form, go to the iContact app and click on Customize.
In the lightbox, scroll down to the Custom Fields subsection. Use the dropdowns on the right to associate form fields with iContact custom fields on the left. You’ll find here all the custom fields built in your iContact account.
Hit Save and you are done. From now on, each new submission will send form data to your custom fields in iContact.
Note: If you change the name (not the label) of a custom field in iContact, you’ll need to return to the 123FormBuilder – iContact app to associate it again to your form field.