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.

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 Access Token.

Click on the Submit button.
​

Select the Constant Contact List you want.