On your wiki, edit the page MediaWiki:Chat-welcome-message. You must do two things to this page. Note that if you have a wiki in a non-English language, you may want to follow instructions for non-English wikis below.
The first thing you need to do is copy-paste the following code to the end of the page:
And replace "YOURWIKI" with whatever wiki you are making the page on.
The second thing you need to do is replace the "$1" with the name of your wiki.
You should end up with something like the picture to the right.
For non-English wikis
Do as is explained above, however also add it to the page MediaWiki:Chat-welcome-message/lang where "lang" is the prefix for your language, like "es" for Spanish or "it" for Italian, so the page would be MediaWiki:Chat-welcome-message/es or MediaWiki:Chat-welcome-message/it respectively.
Now create the page MediaWiki:Chat.js/load.js on your wiki. Copy-paste the following code to that page.
Another way of making personal changes to chat is via your global.js and global.css pages on Community Central. Note that while global.css is loaded by default, global.js must be loaded via other scripts.