Generate autocomplete url of form element

Hey,
I have an entity reference form element with autocomplete widget (some options skiped):

$element = array(
  "target_id" => array(
    "#title" => "My autocomplete field",
    "#type" => "entity_autocomplete",
    "#target_type" => "taxonomy_term",
    "#selection_handler" => "default:taxonomy_term",
    "#selection_settings" => array(
      "target_bundles" => array(
        "tags" => "tags",
      ),
    ),
    "#autocreate" => array(
      "bundle" => "tags",
      "uid" => 1,
    ), 
    "#tags" => 1,
  ),
);

How I can generate an autocomplete url of this form element? For example, in Drupal 7, this url is being in #autocomplete_path option of element.