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.cgi?mode=updinfo&pass=<your password>&song={{artist}} - {{title}}

(replacing <your server> <your port> and <your password> with the valid values for your streaming server)

Leave Username and Password fields empty.

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.