code: @PostConstruct
      public void invokeSlackWebhook() {
          RestTemplate restTemplate = new RestTemplate();
          RichMessage richMessage = new RichMessage("Just to test Slack's incoming webhooks.");
          // set attachments
          Attachment[] attachments = new Attachment[1];
          attachments[0] = new Attachment();
          attachments[0].setText("Some data relevant to your users.");
          richMessage.setAttachments(attachments);
  
          // For debugging purpose only
          try {
              logger.debug("Reply (RichMessage): {}", new ObjectMapper().writeValueAsString(richMessage));
          } catch (JsonProcessingException e) {
              logger.debug("Error parsing RichMessage: ", e);
          }
  
          // Always remember to send the encoded message to Slack
          try {
              restTemplate.postForEntity(slackIncomingWebhookUrl, richMessage.encodedMessage(), String.class);
          } catch (RestClientException e) {
              logger.error("Error posting to Slack Incoming Webhook: ", e);
          }
      }
docs: Make a POST call to the incoming webhook url.