There are a couple of ways you can make this edit. You can either open up the rs and go into advanced view to change the default value for the colname parameter or you can change it directly in the code.
Either way you will need to update -1 value to be an id for the default location in your recordset. Here is the code that you would change directly on the page:
$colname_rstDestination = "-1"; //change the -1 to the id of the default location
if (isset($_POST['location'])) {
$colname_rstDestination = (get_magic_quotes_gpc()) ? $_POST['location'] : addslashes($_POST['location']);
}