To integrate with the MailChimp, you have to use API key. Each MailChimp account has its own set of unique API keys that tell MailChimp who you are and ensure that you have permission to access the requested information.
To find your API key, click on your name on the left. When the slide-out menu appears, choose “Account”:
On the account page, click on "Extras" and then choose "API keys":
The API keys page will list all of your available API keys. To create a new key, scroll down to the bottom and click "Create A Key":
Once your API key created, copy your key:
Then go to the “Integrations” page, click MailChimp and copy your API key in the field and click "Save"::
Choose your signal and click “Edit”:
Then click "Mailchimp integration“, choose your mailing list and click "Save":