Drupal form widget to preset entity ref with current user.

CurrentUserWidget.php

<?php

namespace Drupal\se_core\Plugin\Field\FieldWidget;

use Drupal;
use Drupal\Component\Utility\Color;
use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\Plugin\Field\FieldWidget\EntityReferenceAutocompleteWidget;
use Drupal\Core\Field\WidgetBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\se_core\Seao;
use Drupal\user\Entity\User;

/**
 * Plugin implementation of the 'current_user' widget.
 *
 * @FieldWidget(
 *   id = "current_user",
 *   module = "se_core",
 *   label = @Translation("Current User"),
 *   field_types = {
 *     "entity_reference"
 *   }
 * )
 */
class CurrentUserWidget extends EntityReferenceAutocompleteWidget {

  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $element = parent::formElement($items, $delta, $element, $form, $form_state);
    $account = User::load(Drupal::currentUser()->id());
    if (empty($element['target_id']['#default_value'])) {
      $element['target_id']['#default_value'] = $account;
    }

    return $element;
  }

}