docs: def post_message(channel,
                   message,
                   username,
                   api_url,
                   hook):
      '''
      Send a message to a mattermost room.
  
      :param channel:     The room name.
      :param message:     The message to send to the mattermost room.
      :param username:    Specify who the message is from.
      :param hook:        The mattermost hook, if not specified in the configuration.
      :return:            Boolean if message was sent successfully.
      '''
  
      parameters = dict()
      if channel:
          parameters['channel'] = channel
      if username:
          parameters['username'] = username
      parameters['text'] = '```' + message + '```'  # pre-formatted, fixed-width text
      log.debug('Parameters: %s', parameters)
      result = salt.utils.mattermost.query(
          api_url=api_url,
          hook=hook,
          data=str('payload={0}').format(salt.utils.json.dumps(parameters)))  # future lint: disable=blacklisted-function
  
      log.debug('result %s', result)
      return bool(result)