To set up the Now Playing plugin to push metadata updates to an Icecast server you can configure the HTTP Web Request tab (ticking the Send tickbox), selecting the Method GET and entering the URL 

http://<your server>:<your port>/admin/metadata?mode=updinfo&mount=/<your mount>&song={{artist}} - {{title}}

(replacing <your server> <your port> and <your mount> with the valid values for your streaming server - make sure to keep the / in front of the mount)

The username and password are the same as you use for streaming, for example, on the Internet Broadcast plugin. Enter source as the username if you have left this blank on the encoder settings.

Click Apply Changes. After updating this information you will need to advance to the next track to push that update to the server - if all is good you should see 200 OK on this box. If you have entered incorrectly it will say 401 Unauthorized or 400 Bad Request.