Unable to programmatically render webform with supplied default values
Did you try using the Webform render element documented at the bottom of https://www.drupal.org/docs/8/modules/webform/webform-cookbook/how-to-embed-a-webform.

return [
  '#type' =&gt; 'webform',
  '#webform' =&gt; 'your_form_id',
  '#default_data' =&gt; ['name' =&gt; 'Custom Name'],
];