Add a sub token for a node field

extra_field_token.php

<?php

/**
 * Implements hook_token_info_alter()
 */
function MODULE_token_info_alter(&$data) {
  // Modify description of node tokens for our site.
  $data['tokens']['node-field_myfield']['foo'] = [
    'name' => "Foo",
    'description' => "Process field_myfield value"
  ];

}


/** 
* Implements hook_tokens() 
*/
function MODULE_tokens($type, $tokens, array $data, array $options, \Drupal\Core\Render\BubbleableMetadata $bubbleable_metadata) {
  // $token_service = \Drupal::token();

  $replacements = array();
  if ($type == 'node' && !empty($data['node'])) {
    $node = $data['node'];
    foreach ($tokens as $name => $original) {
      switch ($name) {
        case 'field_myfield:foo':
          if ($node->hasField('field_myfield')) {
            // Do your magic foo processing
            $replacements[$original] = foo($node->field_episode_order->value);
          }
          break;
      }
    }

  }
  return $replacements;
}