How can I enforce a default machine name prefix?

By adding this code to a module, administrators can create forms that will can be exported to config while other users with permissions to create webforms will automatically have the machine name set with the site_ prefix.

function mymodule_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  // Adds validation to webform in order to check machine name on create.
  if ($form_id == "webform_add_form") {
    $form['#validate'][] = 'webform_create_validation';
  }
}

function webform_create_validation(&$form, FormStateInterface $form_state) {
  $current_user = \Drupal::currentUser();
  // If user creating the webform is not an administrator.
  if (!in_array('administrator', $current_user->getRoles())) {
    // Get machine name of webform being created.
    $ent = $form_state->getFormObject()->getEntity();
    if (isset($ent)) {
      $id = $ent->id();
      // Check to see if webform machine name starts with "site_".
      // If it does not, add it and save.
      if (substr($id, 0, 5) !== "site_") {
        $new_id = 'site_' . $id;
        $form_state->setValue('id', $new_id);
      }
    }
  }
}