The “Custom” form action type allows you to hook a WordPress action into form processing.
The “Hook Tag” setting specifies what WordPress action hook to call during the submission process.
During submission the specified “Hook Tag” is passed a $form_data array variable. This array contains the form id, form fields, and form settings data as top level keys in the array.
Notice: The $ninja_forms_processing super global has been removed. All form data is available via the passed $form_data parameter.
Note that the callback cannot update the submitted form data as this is a WordPress action and not a filter. The callback method should be a void return method.