You need to have MailPoet (a free plugin) installed on your site. You can install this through the WordPress admin.
Adding a MailPoet action to your form
In your form editor, click the “Email and Actions” tab.
Click “Add New Action” and then Choose MailPoet
A MailPoet Signup field will be added to your form automatically. And if someone checks that box they’ll be added to your newsletter. You just need to tell Ninja Forms which fields you want to send to MailPoet.
First make sure you select your list. If you don’t see your list you can click the refresh icon.
As soon as you select your list you should see the List Field Mapping fields appear. Now you can click on the icon on the right side of each of those fields and select the best field.
Once you’re happy with your fields. You can click Done. And then Publish or Update your form.
You can now test your form. After you test it you can go to MailPoet in your admin and see the subscriber added to your list.