title: Unable to get timestamp of the message posted by Slack App
body: <p>Whenever my app posts ephemeral message to Slack channel (in response to a query by a user), I am unable to get the timestamp of my Slack app response. As I want to delete it once the user has made a selection using one of the buttons. Although I have subscribed to 'message.channels' event, I don't get a notification to my app whenever my app posts in the channel (in response to the user input), therefore, I am unable to get the timestamp of the message which I'll use to delete it. All I want is the timestamp of the message posted by my app so that I can delete it but I am unable to receive the timestamp. Please help!</p>
  
  <p>For e.g. in Giphy app for Slack. Let's say the user invokes the app by calling '/giphy [dog]' where 'dog' is just an example of a search term. The app responds by sending a gif and user can either send it, shuffle to the next one or cancel it. I want a similar capability of cancelling the app response but I need the timestamp of the message in order to do so therefore I am asking for help.</p>
  
  <p>Thanks.</p>