title: How to send message to Slack listening channels?
body: <p>i am working on a slack app (scope including bot and incoming webhooks). I can send message to a defined channel but i don't know how to stop using the "&amp;channel=" parameter and just send messages to listening channels. By listening i mean, when the app is installed, user is asked where to post (channel or dm has to be chosen). </p>
  
  <pre><code>String postUrl = "https://slack.com/api/chat.postMessage?token=" + botAccessToken + "&amp;as_user=false&amp;channel=%23community&amp;text=testing&amp;username=CommunityQ";
  </code></pre>
  
  <p>Any hints would be useful.</p>