Have a look at this :
https://github.com/osTicket/osTicket-1.7/blob/develop/include/staff/ticket-edit.inc.php(https://github.com/osTicket/osTicket-1.7/blob/develop/include/staff/ticket-edit.inc.php)
On line 7 : <?php csrf_token(); ?>
Try adding this line into your code :)