code: def post_reaction(self, channel_id, timestamp, reaction_name, **kwargs):
          
          params = {
              "post_data": {
                  "name": reaction_name,
                  "channel": channel_id,
                  "timestamp": timestamp,
              }
          }
          params["post_data"].update(kwargs)
  
          return self.api_call("reactions.add", **params)
docs: Send a reaction to a message using slack Event API