Certain embodiments involve generating a real-time notification to facilitate the delivery of customized content, based on detecting that a subject activity occurs within a customized subject region. For instance, a computing system updates a graphical interface to display, as a layer on a map, detected instances of a subject activity performed within a geographic area of the map. The computing system determines a polygon corresponding to a region of the geographic area, where the polygon is determined from a graphical input representing lines at locations on the map on which the detected instances are overlaid. The computing system determines that a location of a detected instance of the subject activity performed by a user device falls within the polygon. The computing system transmits a notification to a content provider, such that the content provider delivers customized content to the user device that performed the detected instance of the subject activity.