Hi Selva,

I would suggest to try the following:
1. Check if getting the same URL (i.e. “https://slack.com/api/rtm.start?token=” + the actual slack token) works for you when you do it from the browser
2. Verify that the ESP is able to connect through the WiFi by performing a GET request to another server (e.g. http://www.example.com)

Good luck with your project!