<?php
public function buildForm(array $form, FormStateInterface $form_state) {
 $config = $this->config('custom_module');
 // Define the element(s).
 $form['nodes'] = [
   '#type' => 'entity_autocomplete',
   '#title' => $this->t('Select favorite articles.'),
   '#multiple' => 15,
   '#target_type' => 'node',
   '#selection_handler' => 'default:node',
   '#selection_settings' => [
     'target_bundles' => ['article' => 'article'],
     'sort' => ['field' => 'title', 'direction' => 'ASC'],
   ],
   '#default_value' => $config->get('nodes'),
 ];
 // Process the element(s).
 $this->elementManager->processElement($form['nodes']);
 // Attach behaviors. Unsaved: Warn users about unsaved changes.
 $form['#attributes']['class'][] = 'js-webform-unsaved';
 $form['#attached']['library'][] = 'webform/webform.form.unsaved';
 return parent::buildForm($form, $form_state);
}