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

<button onclick="closePresentation">
  Exit
</button>
function closePresentation() {
    if (mobilelocker) {
        mobilelocker.closePresentation()
            .then(() => {
                console.log('Presentation closed successfully')
            })
            .catch(err => {
                console.error('Presentation could not be closed successfully', err.message);
            })
            .finally(() => {
                // @see https://developer.mozilla.org/en-US/docs/Web/API/Window/close
                window.close();
            })
    }
}

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

You can also close the current presentation programmatically:

mobilelocker.closePresentation();
<a href="mobilelocker/api?method=close-presentation">Close presentation</a>