Embedding a Vanilla Forum as a Drupal block

VanillaForumBlock.php

<?php

// Put this file in YOUR_MODULE/src/Plugin/Block.

namespace Drupal\YOUR_MODULE\Plugin\Block;

use Drupal\Core\Block\BlockBase;

/**
 * Provides the Vanilla Forum Block.
 *
 * @Block(
 *   id = "vanilla_forum",
 *   admin_label = @Translation("Vanilla Forum"),
 *   category = @Translation("Forum"),
 * )
 */
class VanillaForumBlock extends BlockBase {

  /**
   * {@inheritdoc}
   */
  public function build() {
    // The site hosting the forum may be specified as a configuration setting.
    $vanilla_root = \Drupal::config('vanilla.settings')->get('vanilla_url')) {
    $ext_js = '<script type="text/javascript" src="' . $vanilla_root . '/js/embed.js"></script>';

    return [
      '#markup' => $ext_js,
       // Without #allowed_tags the <script> tag will be filtered out during rendering.
      '#allowed_tags' => ['script'],
    ];
  }

}