Step 1: Log in to Constant Contact
On the Run form builder, click to add destination, and after you select Constant Contact, click the button to login. After you've successfully logged in, click the Save button. Confirm that it's connecting correctly under Status after you save.
Step 2: Select Constant Contact as the destination on ALL forms you plan to use
On the form builder, you must click on the Enable checkbox to the left of the destination for all forms you want to use this destination for on the site.
If you have any Lists on your Constant Contact account, they'll appear in your Run form builder upon successful connection. You can select a List and then any submissions that come through the form will be added to that List on Constant Contact.
In order to add custom fields to your form, you'll first need to add those fields on Constant Contact. Make sure to select Text Field as your Field Type.
Then, open your Run form builder, click Add Form Field, and add a Custom Field (like 'Text' for example). Under the Constant Contact custom fields column, Run will pull down a list of your custom fields from Constant Contact to select from.