Issue #3101563 by JeroenT, mizutaniDC: Add a trailing slash to the path...

      '#default_value' => $default_url_value,

      '#maxlength' => 2048,

      '#required' => $element['#required'],

      '#field_prefix' => Url::fromRoute('<front>', [], ['absolute' => TRUE])->toString(),

      // Add a trailing slash to make it more clear that a redirect should not

      // start with a leading slash.

      '#field_prefix' => \Drupal::request()->getSchemeAndHttpHost() . '/',

      '#attributes' => ['data-disable-refocus' => 'true'],

    ];