Within Teamwork Desk, you can completed a task directly from the ticket it is associated with.
In the Tickets area of your site, select the relevant ticket from the list view. Once you are viewing the ticket, go to the Tasks tab.
You will see a list of any active and completed tasks associated with the ticket. Click the grey checkmark to the left of the task to mark it as complete.
The checkmark will turn green and a line will appear through the task title to indicate it is complete. The task will also show as completed in your Teamwork site.
Note: When a linked task on a ticket is marked as complete, Desk will automatically set the ticket status back to active.
For more information, see: Creating a Task in a Ticket