Webform Confirmation AjaxResponse
I ended up doing this by creating an event subscriber and listening for the webform ajax response as described on this issue page: https://www.drupal.org/project/webform/issues/2911409

The keylines (amended for brevity): 

// The event to listen for
KernelEvents::RESPONSE

// Check to see if it is a webform ajax request 
if(
    $event-&gt;getResponse() instanceof WebformSubmissionAjaxResponse and
    $event-&gt;getResponse()-&gt;getWebformSubmission()-&gt;isCompleted()
) 

// Add my custom Ajax command
$event-&gt;getResponse()-&gt;addCommand(new UrlHistoryCommand)