You would need a page where anonymous users have access to. Make sure that the page can be reached with a direct URL. On the page show a non persistent object that you create with datasource microflow. I would ask more then only the ticket number because of security ( I do not think you want to show each ticket to anybody). Most of the time the ticket may also contain the email address of the person who raised the ticket so that would be a good option also. Then make a search button that triggers a flow that based on the values tries to retrieve the right ticket or give a message in return that the ticket could not be found.
Good luck and regards,
Ronald