The Opt-in Checkbox feature was added to Contact Form 7 MailChimp Extension so you can give your visitors the option to opt-in / opt-out to subscribe to your mailing list(s).
Using the Contact Form 7 MailChimp Extension Opt-in Checkbox feature, is super simple. Take a look a the screenshots below:
To generate a form such as this one below:
Configure your form the same way you see it on this screenshot. Notice this mail tag inside the form: [checkbox my-optin “Subscribe me your newsletter”]
Add this mail-tag [my-optin] to the settings of Contact Form 7 MailChimp Extension as in the screenshot below:
After these steps have been done, save your form and run some tests.