code: protected final void reply(WebSocketSession session, Event event, Message reply) {
          try {
              if (StringUtils.isEmpty(reply.getType())) {
                  reply.setType(EventType.MESSAGE.name().toLowerCase());
              }
              reply.setText(encode(reply.getText()));
              if (reply.getChannel() == null && event.getChannelId() != null) {
                  reply.setChannel(event.getChannelId());
              }
              synchronized (sendMessageLock) {
                  session.sendMessage(new TextMessage(reply.toJSONString()));
              }
              if (logger.isDebugEnabled()) {  // For debugging purpose only
                  logger.debug("Reply (Message): {}", reply.toJSONString());
              }
          } catch (IOException e) {
              logger.error("Error sending event: {}. Exception: {}", event.getText(), e.getMessage());
          }
      }
docs: Method to send a reply back to Slack after receiving an {@link Event}.
  Learn <a href="https://api.slack.com/rtm">more on sending responses to Slack.</a>
  
  @param session websocket session between bot and slack
  @param event   received from slack
  @param reply   the message to send to slack