Drupal 8 Webform URL alias with parameters On the URL PATH SETTINGS,  I want to define the Webform URL alias to include a variable parameter e.g. /my-web-form-url/{id}
Is this possible? I want to pass this 'id' as a hidden value in the form.
You would need to write some custom code to support custom URL parameters. (see How to (truly) alter a URL in Drupal 8)

You could easily pass the 'id' as a query string parameter, which is supported as long as you enable 'Allow elements to be populated using query string parameters.' under a Webform's > Settings > Form Behaviors.