Board Thread:Support Requests - Getting Technical/@comment-24575746-20131122205303/@comment-168424-20131123053635

Short answer. You would probably need to code it in Javascript if you want it to be talk page specific...

If you don't need it to be talk page specific...

You need to be an admin. The message is technically not on all article talk pages but will be seen by any user who visits the wiki.
 * Wiki-wide message

Edit this page to edit the wiki-wide notification popup message: MediaWiki:Communitymessages-notice-msg

An then edit this page to trigger the popup: MediaWiki:Community-corner