Board Thread:Support Requests - Getting Technical/@comment-4146635-20140517234855/@comment-856287-20140520230505

Maybe I made it sound complicated, but it's really not. You edit MediaWiki:Chat-welcome-message and then create MediaWiki:Chat.js/load.js. The instructions are pretty clear. 😉

Once that's done, chat will start looking for a file called MediaWiki:Chat.js (where before it wouldn't load any JS pages by default). You create that page with the code above and it should be all set.