File Content: bootstrap.php <?php mc4wp_register_integration('ninja-forms', 'MC4WP_Ninja_Forms_Integration', true); add_filter('ninja_forms_register_fields', function ($fields) { if (class_exists(NF_Abstracts_Input::class)) { $fields['mc4wp_optin'] = new MC4WP_Ninja_Forms_Field(); } return $fields; }); add_filter('ninja_forms_register_actions', function ($actions) { if (class_exists(NF_Abstracts_Action::class)) { $actions['mc4wp_subscribe'] = new MC4WP_Ninja_Forms_Action(); } return $actions; }); Submit