/**
 * Implements hook_entity_form_mode_alter().
 */
function {{ machine_name }}_entity_form_mode_alter(&$form_mode, \Drupal\Core\Entity\EntityInterface $entity) {
  // Change the form mode for users with Administrator role.
  if ($entity->getEntityTypeId() == 'user' && $entity->hasRole('administrator')) {
    $form_mode = 'my_custom_form_mode';
  }
}