How to display a modal in the beforeunload?

Question

With the event beforeunload, i can display a message when the page is to be canceled or updated.

 $(window).on('beforeunload', function () {
     return 'Testando';
 });

But the client for whom I usually develop systems do not like many of the layouts of the alert, confirm or prompt javascript.

So I’d like to, at this event of beforeunload, put a modal displaying a message in place of that alert appearing.

Is there any way to do this ? Is there any way to use the event beforeunload, without posting the message of alert?