Getting an autocomplete field widget for use in block configuration.

autocomplete-block-widget.php

<?php
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Field\BaseFieldDefinition;
use Drupal\Core\Field\EntityReferenceFieldItemList;
use Drupal\Core\TypedData\DataDefinition;
use Drupal\Core\TypedData\Plugin\DataType\ItemList;

/**
 * Provides a 'FooBlock' block.
 *
 * @Block(
 *  id = "foo_block",
 *  admin_label = @Translation("FooBlock"),
 * )
 */
class FooBlock extends BlockBase {

  /**
   * {@inheritdoc}
   */
  public function blockForm($form, FormStateInterface $form_state) {
    $widget_plugin_manager = \Drupal::service('plugin.manager.field.widget');

    $base_field_definition = BaseFieldDefinition::create('entity_reference')
      ->setLabel(t('Node Reference'))
      ->setDescription(t('The Node.'))
      ->setSetting('target_type', 'node')
      ->setSetting('handler', 'default')
      ->setName('field_reference')
      ->setCardinality(1);

    $widget_options = array(
      'field_definition' => $base_field_definition,
      'form_mode' => 'default',
      'configuration' => array(
        'type' => 'entityreference_auto_complete',
      ),
    );

    $instance = $widget_plugin_manager->getInstance($widget_options);

    $element = array();

    $item_list = new ItemList(new DataDefinition());
    $list = new EntityReferenceFieldItemList($base_field_definition, 'reference_list', $item_list);
    
    // Fail, `formElement` needs `$list->parent` to be an entity with en entity-reference field.
    $form_field = $instance->formElement($list, 12, $element, $form, $form_state);

    return $form;
  }
}