You will need a Constant Contact developer account to connect the Constant Contact account with Divi's Email Opt-in module.
Go to the Constant Contact developer's page.
Create a Mashery Application by going to this page.
Log in with your Contant Contact developer's account.
Click on the Applications Tab and Create a new Application.
Click on the Register Application button.
Go to the API Tester Tab and click on the Get Access Token button.
Copy the Token.
Connect Constant Contact with Divi's Email Opt-in module
Open the Email Opt-in module Settings modal window.
Go to the Content Tab → Email Account.
Select Constant Contact from the provider's list.
Click on the Add button.
Give it a name. It can be anything you want.
Paste in the API and Acess Token.
Click on the Submit button.
Select the Constant Contact List you want.