I'm not quite sure what you're looking to do.

The Email Button (introduced in Juicebox-Pro v1.5.0) is essentially a mailto link which, when clicked, opens the user's default email program to send an email. Clicking the Email Button does not open a new web page or browser tab.

If you want to override Juicebox-Pro's own functionality for the Email Button and you want the Email Button to do something else instead, then you'd need to override it using JavaScript code such as the following:

<script src="jbcore/juicebox.js"></script>
<script>
    var jb = new juicebox({
        containerId: "juicebox-container",
        showEmailButton: "TRUE"
    });
    jb.onInitComplete = function() {
        $('.jb-bb-btn-email').off('click');
        $('.jb-bb-btn-email').click(function() {
            // Custom JavaScript code to do something goes here
        });
    };
</script>

I hope this helps.