code: public function useProxy($proxy, $username = false, $password = false)
      {
          $this->user_agent->useProxy($proxy, $username, $password);
      }
docs: Sets proxy to use on all requests for when testing from behind a firewall.
  Set URL to false to disable.
  
  @param string $proxy        Proxy URL.
  @param string $username     Proxy username for authentication.
  @param string $password     Proxy password for authentication.