Lex bot not responding to messages sent through Slack's chat.postMessage API

Question

I am building a lex bot that responds to messages on slack channel. The bot is responding perfectly to messages typed by user in that channel but if the message is send using the chat.postMessage API, then the lex bot is not giving any response.

I tried the same with azure bot and it worked perfectly. Any suggestions?

This is the api call I am using:

request('slack.com/api/chat.postMessage?token='+token+'&channel=DBH3EAK1Q&text=hello&as_user=true', 
    function (error, response, body) { 
      console.log(body);
    }
});