This provides a taken to access the site's base URL. See: https://www.drupal.org/node/1088112

base_url_token.module

/**
 * Implements hook_token_info_alter().
 */
function base_url_token_token_info_alter(&$data) {
  if (isset($data['tokens']['site'])) {
    $data['tokens']['site']['base-url'] = [
      'name' => t("Base URL"),
      'description' => t("The site's base URL"),
    ];
  }
}

/**
 * Implements hook_tokens().
 */
function base_url_token_tokens($type, $tokens, array $data, array $options, \Drupal\Core\Render\BubbleableMetadata $bubbleable_metadata) {
  $replacements = array();
  if ($type == 'site') {
    foreach ($tokens as $name => $original) {
      switch ($name) {
        case 'base-url':
          $base_url = \Drupal::request()->getSchemeAndHttpHost();
          $replacements[$original] = $base_url;
          break;
      }
    }
  }
  return $replacements;
}