Well, you will replace that eventually (or rather get rid of that PINGPONG message altogether) once it is working... I think it is working right now, based on your Construct code. It doesn't send your PING message because of the code. You specifically tell it to ONLY send the message IF the message equals EXACTLY: "PING :tmi.twitch.tv", so if you just type in "PING", it will fail to send that message.
It will still send your other message (the sendInput.Text one) though, which it does, but only when you click the button (not if you press enter).