Within Teamwork, you can associate a task list on a project with an individual milestone.
Go to the List area of your project and hover over the name of the relevant task list. Click the three dots beside the task list name and select Edit List from the dropdown menu.
In the pop-up modal, select the Milestone tab. You can then choose an existing milestone from the dropdown, or create a new one. Click Save Changes to add the milestone to the task list.
The milestone will be attached to the task list and appear below the task list name in your project's List area.
Note: Any tasks in the task list that do not have their own due date set will automatically inherit the due date of the milestone.
For more information, see: Adding a Milestone