public static function addMoreAjax(array $form, FormStateInterface $form_state) {
$button = $form_state
->getTriggeringElement();
// Go one level up in the form, to the widgets container.
$element = NestedArray::getValue($form, array_slice($button['#array_parents'], 0, -2));
// Add a DIV around the delta receiving the Ajax effect.
$delta = $element['#max_delta'];
$element[$delta]['#prefix'] = '<div class="ajax-new-content">' . (isset($element[$delta]['#prefix']) ? $element[$delta]['#prefix'] : '');
$element[$delta]['#suffix'] = (isset($element[$delta]['#suffix']) ? $element[$delta]['#suffix'] : '') . '</div>';
return $element;
}