How to change MessageLabel on the Introduction Page?

[Sinbad]

From the documentation:

Controller.prototype.IntroductionPageCallback = function()
{
    var widget = gui.currentPageWidget(); // get the current wizard page
    if (widget != null) {
        widget.title = "New title."; // set the page title
        widget.MessageLabel.setText("New Message."); // set the welcome text
    }
}

But it doesn't work properly. First I see standard message, then if I press Next, then Back again I see the new message.
I need to set my custom message or just remove the Introduction Page at all.