cms_content_sync-3.0.x-dev/modules/cms_content_sync_custom_field_example/src/Plugin/Field/FieldWidget/CustomFieldWidget.php

<?php

namespace Drupal\cms_content_sync_custom_field_example\Plugin\Field\FieldWidget;

use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\WidgetBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Plugin implementation of the 'CustomFieldWidget' widget.
 *
 * @FieldWidget(
 *   id = "cs_custom_field_widget",
 *   label = @Translation("Content Sync - Custom Field Widget"),
 *   field_types = {
 *     "cs_custom_field"
 *   }
 * )
 */
class CustomFieldWidget extends WidgetBase {

  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $element['cs_text_field'] = [
      '#title' => $this->t('Content Sync - Text Field'),
      '#type' => 'textarea',
      '#default_value' => $items[$delta]->cs_text_field ?? NULL,
    ];

    return $element;
  }

}