Question
How can I add the ticket ID to the ticket subject?
Answer
Adding the ticket ID to the subject of a ticket can be done through the use of business rules.
Although it is not possible to completely modify the subject of a ticket via business rules, it is possible to add information to the email subject sent to users for ticket updates through email. This can be done by adding the placeholder {{ticket.id}} in the subject line of a trigger or automation. The placeholder adds the ticket ID to outgoing email subjects for ticket updates sent to end users.
For more information on placeholders, see the article: Zendesk Support placeholders reference.