In the specific task, click next to Tags and choose the tags to be added from the dropdown. To remove a tag, click on the x for the tag to remove it.
Users will need view and edit permissions in tasks to add tags to a task. Project owners and users with permission to configure project settings can create custom tags that and make them available in the Settings tab. When adding a tag, they can also create a new tag by entering it in the top of the pop up window (where it reads Add new tag...) and select Save.