your form is using the POST method, so the code for the trigger:
if((isset($_GET['UserID']) && $_GET['UserID'] != "")) {
will not work, it needs to use the post method also:
if((isset($_POST['UserID']) && $_POST['UserID'] != "")) {
the value you use for setting the session, is the form post value:
$_SESSION["UserID"] = $_POST['UserID'];