def auth(self, request):
        """
            let's auth the user to the Service
            :param request: request object
            :return: callback url
            :rtype: string that contains the url to redirect after auth
        """
        request_token = super(ServiceTrello, self).auth(request)
        callback_url = self.callback_url(request)

        # URL to redirect user to, to authorize your app
        auth_url_str = '{auth_url}?oauth_token={token}'
        auth_url_str += '&scope={scope}&name={name}'
        auth_url_str += '&expiration={expiry}&oauth_callback={callback_url}'
        auth_url = auth_url_str.format(auth_url=self.AUTH_URL,
                                       token=request_token['oauth_token'],
                                       scope=self.scope,
                                       name=self.app_name,
                                       expiry=self.expiry,
                                       callback_url=callback_url)

        return auth_url