As soon as a ticket is created, a ticket ID will be generated for each ticket in the Helpdesk. If the customer wishes to send an acknowledgment email to the requester with the ticket ID, then the same can be set up from under admin>>email notification>>requester notification,
The below placeholder can be used to send the ticket ID to the customers - [#{{ticket.id}}].
Apart from acknowledgment emails, it is also advisable to use the above ticket ID placeholder in the specified format in the agent reply template to avoid ticket threading issues as ticket ID is one of the primary checks for threading.