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.
You don't have to have the Formidable Pro plugin, it works with the regular Formidable Forms plugin as well.
View the installation and set-up documetation for details.
Features of this WordPress Newsletter Plugin Extension
A "Newsletters" field is added to the Formidable plugin to instantly add a subscribe checkbox to any form.
Modify each "Newsletters" subscribe checkbox individually: auto checking it, assigning lists and mapping fields from the form to custom fields.
A subscribe checkbox is generated by the "Newsletters" field so that your users can subscribe while submitting a Formidable form.