Note, only admins are able to create Assigned Areas.
Step 1: On https://publicsafety.ring.com/assigned_areas, click on the symbol on the top-left corner of the map.
Step 2: Click on the map to start drawing your shape. Double-click to end drawing your shape.
Step 3: Click the blue check-mark button on the bottom of the page.
Step 4: Name this Assigned Area to complete the creation process.