public function userLogin($username, $password)
    {
        return $this->requestHandler->sendRequestAsync('POST', 'api/v4/users/login/',
            [
                'form_params' => [
                    'username' => $username,
                    'password' => $password
                ]
            ])->then(
            function ($result) {
                $this->requestHandler->setAccessTokenCredentials($result['tokenKey'], $result['tokenSecret']);
                return $result;
            }
        );
    }