The Newsletter plugin has integration into the Formidable plugin with the purpose of capturing subscribers through formidable forms.
When creating/editing a Formidable form, a “Newsletters” field is available which works like the other fields but this one instantly creates a subscribe checkbox field. Under “Field Options” the field has options to auto check the checkbox, specify which list(s) the subscribers should be added to and best of all, mapping fields from the Formidable form to custom fields in the Newsletter plugin.
When a user is browsing on your website and goes to one of your forms where you added this “Newsletters” field to the Formidable plugin form, a subscribe checkbox will be displayed and if it is or remains checked as the form is submitted, the person is subscribed to your Newsletter plugin with the specified settings.
View the installation and set-up documetation for details.
See a video overview of this add-on: