Search API Event Suscriber to update custom value

EventSearchapi.php

<?php

declare(strict_types=1);

namespace Drupal\ecm_solr\EventSubscriber;

use Drupal\search_api\Event\SearchApiEvents;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Drupal\search_api\Event\IndexingItemsEvent;

/**
 * @todo Add description for this subscriber.
 */
final class EcmSolrSubscriber implements EventSubscriberInterface {

  public function indexingItems(IndexingItemsEvent $event): void {
    //this can be used to change the stage as well
    $index = $event->getIndex();
    $items = $event->getItems();
    foreach ($items as $item) {
      $fields = $item->getFields();
      foreach ($fields as $field) {
        //Get field name
        //Check here if ts & ss can be appended

        $fieldName = $field->getFieldIdentifier();
        //value got updated here
        if ($fieldName == 'jackson_string'){
          $field->setValues(['New John Value']);
        }
      }
    }

  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents(): array {
    $events[SearchApiEvents::INDEXING_ITEMS][] = ['indexingItems'];
    return $events;
  }


}