If you want to add a custom EXIT button to your HTML, do something like this:

<button onclick="closePresentation">
  Exit
</button>
function closePresentation() {
    if (confirm('Are you sure you want to exit the presentation?')) {
        mobilelocker.closePresentation()
    }
}

The forms-demo project has an example of how this works. Look at the exit() function.

You can also close the current presentation programmatically:

if (confirm('Are you sure you want to exit the presentation?')) {
    mobilelocker.closePresentation();
}
<a href="mobilelocker/api?method=close-presentation">Close presentation</a>