This type of info is usually in the owner's manual. I downloaded every owner'manual for all Agility 125s and one variant and none have an oil service light. There is no freebee download for the 16+. A bike purchased new should have an owner's manual. See your dealer for one. However, the Downtown 300i has this kind of light as do other models and it is activated by the odometer, specifically, one of the odometers. Can you select the main odometer and a trip odometer? If you can, I expect there is an odometer that has no tag or name like "ODO" or "TRIP" it will just show a number. Like the DT300i it should reset by pushing both Mode and Adjust buttons at the same time and holding until the number sets to "0.0" and this will make the light go dark. It will come back on in about 600 miles. I use mine for a gasoline odometer just so that darn light stays out! After the initial oil changes the interval on the DT is 3000 miles so the 600 mile light is nonsense anyway. It IS REAL NEAT to have 4 odometers!
Karl