To integrate with Ninja Forms, add the “Mailchimp” action to one of your Ninja Forms forms by activating it in the Emails & Actions tab of the Ninja Forms form editor.
Once the MailChimp action is activated you can click the gear icon to go to the relevant settings and set a list for this integration, as well as turning on or off double opt-in and setting it to update existing subscribers or not.
Once the list is selected you can map the fields from your Ninja Form to the fields in your MailChimp list.
If you want you can also add the checkbox to the form to make the newsletter subscription optional. If you do not add this checkbox to the form, anyone who uses the form will be subscribed to the selected MailChimp list.
Note that adding the checkbox to the form on its own won’t do anything, you also need to add the MailChimp action to the form!