<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def post_chat_message(self, merchant_id, channel_id, message):
 """post a chat message Arguments: channel_id: Scan token """


        return self.do_req('POST',
                           self.base_url + '/chat/v1/merchant/%s/channel/%s/message/' % (merchant_id, channel_id),
                           message)