You will need a Campaign Monitor API key to connect your Campaign Monitor account.
Get the required Data
Log in to your Campaign Monitor account
Click on your profile picture at the top right corner
Choose Account Settings.
Click on API Keys.
Click on the Generate API key and fill in the login password.
Connect Campaign Monitor with Divi's Email Opt-in module
Open the Email Opt-in module Settings modal window.
Go to the Content Tab → Email Account.
Select the Campaign Monitor from the provider's list.
Set a name for your account. This can be anything you want. You can use, for example
my-campaign-monitor
Paste the Campaign Monitor API Key
Click on the Submit button.
Choose the Campaign Monitor list you want to use.