AHAH callback is not working
Neither #ahah nor path are used in Drupal 8. Instead of the first, Drupal 8 uses #ajax; instead of the second, it uses url which must be a \Drupal\Core\Url to which to submit the Ajax request.

Finally, callback is a PHP callback, which generally is a full qualified (using a PSR-4 namespace) method name.

Reference


Ajax API
PSR-4 namespaces and autoloading in Drupal 8