Ninja Forms Custom Merge Tags

_plugin.php

<?php

/*
 * Register the new merge tag class on the `ninja_forms_loaded` hook.
 */

add_action( 'ninja_forms_loaded', 'my_register_merge_tags' );
function my_register_merge_tags(){
  require_once 'class.mergetags.php';
  Ninja_Forms()->merge_tags[ 'my_merge_tags' ] = new My_MergeTags();
}

class.mergetags.php

<?php

class My_MergeTags extends NF_Abstracts_MergeTags
{
  /*
   * The $id property should match the array key where the class is registered.
   */
  protected $id = 'my_merge_tags';
  
  public function __construct()
  {
    parent::__construct();
    
    /* Translatable display name for the group. */
    $this->title = __( 'My Merge Tags', 'ninja-forms' );
    
    /* Individual tag registration. */
    $this->merge_tags = array(
      
        'foo' => array(
          'id' => 'foo',
          'tag' => '{my:foo}', // The tag to be  used.
          'label' => __( 'Foo', 'my_plugin' ), // Translatable label for tag selection.
          'callback' => 'foo' // Class method for processing the tag. See below.
      ),
    );
    
    /*
     * Use the `init` and `admin_init` hooks for any necessary data setup that relies on WordPress.
     * See: https://codex.wordpress.org/Plugin_API/Action_Reference
     */
    add_action( 'init', array( $this, 'init' ) );
    add_action( 'admin_init', array( $this, 'admin_init' ) );
  }
  
  public function init(){ /* This section intentionally left blank. */ }
  public function admin_init(){ /* This section intentionally left blank. */ }
  
  /**
   * The callback method for the {my:foo} merge tag.
   * @return string
   */
  public function foo()
  {
    // Do stuff here.
    return 'bar';
  }

}