foo.php

<?php

namespace Drupal\foo_video\Plugin\Field\FieldWidget;

use Drupal\Core\Entity\FieldableEntityInterface;
use Drupal\Core\Field\FieldDefinitionInterface;
use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\WidgetBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Form\OptGroup;

/**
 * Provides a options widget that allows that selects the first available option.
 *
 * @FieldWidget(
 *   id = "hidden_option_first_value",
 *   label = @Translation("Hidden option first value"),
 *   field_types = {
 *     "entity_reference",
 *   },
 * )
 */
class HiddenOptionFirstValue extends WidgetBase {

  /**
   * abstract over the actual field columns, to allow different field types to
   * reuse those widgets.
   *
   * @var string
   */
  protected $column;

  /**
   * {@inheritdoc}
   */
  public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, array $third_party_settings) {
    parent::__construct($plugin_id, $plugin_definition, $field_definition, $settings, $third_party_settings);
    $property_names = $this->fieldDefinition->getFieldStorageDefinition()->getPropertyNames();
    $this->column = $property_names[0];
  }

  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $entity = $items->getEntity();
    $options = $this->getOptions($entity);

    $element['target_id'] = $element + [
      '#type' => 'value',
      '#value' => (string) reset($options),
    ];

    return $element;
  }

  public function massageFormValues(array $values, array $form, FormStateInterface $form_state) {
    return $values;
  }

  public function extractFormValues(FieldItemListInterface $items, array $form, FormStateInterface $form_state) {
    return parent::extractFormValues($items, $form, $form_state);
  }


  /**
   * Returns the array of options for the widget.
   *
   * @param \Drupal\Core\Entity\FieldableEntityInterface $entity
   *   The entity for which to return options.
   *
   * @return array
   *   The array of options for the widget.
   */
  protected function getOptions(FieldableEntityInterface $entity) {
    if (!isset($this->options)) {
      // Limit the settable options for the current user account.
      $options = $this->fieldDefinition
        ->getFieldStorageDefinition()
        ->getOptionsProvider($this->column, $entity)
        ->getSettableOptions(\Drupal::currentUser());

      $module_handler = \Drupal::moduleHandler();
      $context = [
        'fieldDefinition' => $this->fieldDefinition,
        'entity' => $entity,
      ];
      $module_handler->alter('options_list', $options, $context);

      // Options might be nested ("optgroups"). If the widget does not support
      // nested options, flatten the list.
      $options = OptGroup::flattenOptions($options);

      $this->options = $options;
    }
    return $this->options;
  }

}