ninja-forms-three-newsletter-action.php

<?php
final class NF_Example_Actions_Newsletter extends NF_Abstracts_ActionNewsletter {
  
  // ...
  
  protected function get_lists()
  {
    return array(
      array(
        'value' => 1,
        'label' => 'One',
        'fields' => array(
            array(
                'value' => 'one',
                'label' => __( 'Field One', 'ninja-forms-mail-chimp' )
            ),
          )
        ),
      );
  }
}