I don’t think you can do it directly. Some thoughts:
1. Have you tried putting the code in the On Change event of the multipage?
if (activePage == 'm1-screen-Chat') {
phoneui.showURL('my-url');
}
2.You also could place custom button over the chat tab and make the it transparent. Then use its on Click event to show the URL.
3. You could just place a regular button on the page that says “Click to Chat” or something.