The parent-child related task can be configured in the tickets by using this new feature in service desk.
Step : Go to Service Desk on that select "Admin Panel" it will navigate you to next page.Click Manage button on that choose "List" option prevailed in the admin panel.
Step : Select Add New Custom List by providing the fields name, value and parent list respectively.
For Example: Create a New Custom List as "City" by providing the name in Definition field with the value in items column and select the parent list as existing custom list.Once after selecting the parent list it will load the content related to the parent list which you have selected on that click the appropriate field for displaying it. After providing the values to the list click "save changes" Button.
Step : Select Forms option under the Manage field for linking the list on the Form.For linking the country and city in contact information edit the built-in forms and add the form fields for the ticket information and click Save Changes Button.
Step : For viewing the parent-child linked lists in the ticket.Go to Staff panel → User Directory on that click Add User Button.It will open the create a user button on that it will display the country and city custom fields that linked together while creating the list.