If you signed up before Aug 9, 2021, please click Previous plans to view your applicable plans.
The 'Tag' field will be available under the 'Ticket Properties' panel on the right hand side of the ticket details page. You can either manually type out and create new tags or add existing ones to tickets.
If you are using Freshdesk on Mint, the 'Add tag' option will be available above the main message/description of the ticket, in the ticket details page: