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.