how to get the default value for a multivalue field

Question

I'm using the Mulitvalue Field module (https://www.drupal.org/project/multivaluefield) and I created a form like this and it successfully sets the various emails into the configuration file, but when i try to get the default value it, gets an array back. how do i get the default value for this multivalue field to display it in the form?

$form['comment']['comment_admin_notify_mailto'] = array(
  '#type' => 'multivalue',
  '#title' => $this->t('Comments Topics And Post Approvers'),
  '#cardinality' => MultiValue::CARDINALITY_UNLIMITED,
  'comment_admin_notify_mailto' => [
    '#type' => 'textfield',
    '#title' => $this->t('Person to send notifications for Comments, Topics and Posts'),
    '#default_value' =>  vista_moderation_admin_notify_variable_get_site_email(),
  ],
);

this is the function code:

function vista_moderation_admin_notify_variable_get_site_email() {
 
  $moderation_config = \Drupal::config('vista_moderation_admin_notify.settings');
  $site_mail = $moderation_config->get('comment_admin_notify_mailto');
  if (empty($site_mail)) {
    $site_config = \Drupal::config('system.site');
    $site_mail = $site_config->get('mail');
  }
  return $site_mail;

Answer 1

i found the answer in the comments of the module which is the documentation :

 /**
   * Sets the default value for the child elements.
   *
   * @param array $elements
   *   The elements array.
   * @param array $value
   *   An array of values, keyed by the children element name.
   */
  public static function setDefaultValue(array &$elements, array $value): void {
    // @todo Handle nested elements.
    foreach (Element::children($elements, FALSE) as $child) {
      if (isset($value[$child])) {
        $elements[$child]['#default_value'] = $value[$child];
      }
    }
  }