Back to the 123ContactForm interface, choose the Constant Contact list to which you want to add contacts and click Customize. Match Constant Contact fields with the fields in your 123ContactForm web form. Data will be sent to your Constant Contact list according to these associations.
You can opt to update contacts by email, as well as sending the welcome email on submission. Scroll down the customization lightbox and enable any of the checkboxes below to enable the options.
Press Save in the customization lightbox, then Save again in the main application box.
The integration is now enabled the contacts received through your form will be added automatically to your Constant Contact mailing list.