Add a hidden form element to capture the URL variable and pass it with the form:
<input type="hidden" name="CustomerID" id="CustomerID" value="<?php echo(isset($_GET['CustomerID'])?$_GET['CustomerID']:""); ?>" />
since the form uses the GET method, It takes over the URL Querystring. on form data can be passed in querystring with a form that uses the GET method. Adding a form element to capture the querystring that is passed in will allow you to pass it back out again.