$webform = Drupal::entityTypeManager()->getStorage('webform')->load($machineName);
$viewBuilder = Drupal::service('entity_type.manager')->getViewBuilder('webform');
$formBuild = $viewBuilder->view($webform);
return render($formBuild);

$build = [
  '#type' => 'webform',
  '#webform' => $machineName,
];

$result = new FilterProcessResult($text);
    ...
    // We need to render the embedded entity:
    // - without replacing placeholders, so that the placeholders are
    //   only replaced at the last possible moment. Hence we cannot use
    //   either renderPlain() or renderRoot(), so we must use render().
    // - without bubbling beyond this filter, because filters must
    //   ensure that the bubbleable metadata for the changes they make
    //   when filtering text makes it onto the FilterProcessResult
    //   object that they return ($result). To prevent that bubbling, we
    //   must wrap the call to render() in a render context.
    $entity_output = $this->renderer->executeInRenderContext(new RenderContext(), function () use (&$build) {
      return $this->renderer->render($build);
    });
    $result = $result->merge(BubbleableMetadata::createFromRenderArray($build));