Within Teamwork, you can associate a task list on a project with an individual milestone.
Go to the Table or List area of your project and click the three dots to the right of the relevant task list name, then 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 will be visible below the task list name when in the List area of your project.
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