Use this applet to:
1) Get a ping to your server (This is the equivalent of Passthru applet in voice) - To achieve this, Put a URL in the text box and ensure that the URL returns back a 200 OK with empty document body. But unlike Passthru, You cannot return other response codes to make a boolean decision.
2) To send a static reply - Just type out the text in the box.
3) To send a dynamic reply - Put a URL in the text box and return back 200 OK along with the text that needs to be sent back to the sender.
More details on the query parameters passed as a part of "Send a reply" applet is in the API docs page.