If you have a Fester Detector in your inventory, you can either activate it there or on the Haunted Woods Hunt page. You will be prompted to use it when you visit the Haunted Woods Hunt. You can click the prompt to use it, or you can save it for later.
Once your Fester Detector has been activated, your next click will guarantee you find a super rare item! This will be the first click after activation, or the first click of the day if you activated it previously.