If you use the callback method the session of the user will not be available because the GrabzIt servers are calling the handler and not the user. So the session would be a session for the GrabzIt server not for the user who perhaps initiated the screenshot.
Therefore you need to pass the user id to the custom identifier parameter, which can be read like so:
$customId = $_GET['customid'];
The custom id could for instance be a folder name or perhaps a database id that can be used to work out where the screenshot should be saved.