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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def link_user_to_group(self, user_id, group_id, headers=None, endpoint_url=None):
 """ Links a user to a group :param str user_id: identified user's ID :param str group_id: group ID :param dict headers: custom request headers (if isn't set default values are used) :param str endpoint_url: where to send the request (if isn't set default value is used) :return: Response """

        endpoint_url = endpoint_url or self._endpoint_url

        url = endpoint_url + '/groups/' + group_id + '/link/' + user_id
        headers = headers or self._default_headers()

        response = requests.post(url, headers=headers)

        return response