You simply want to add the boundaries of your city/municipality to a map inside your folder but you don't know where to find that GeoJson file? Here's a quick tutorial on how you can easily download that file from the free tool Wikimapia.
STEP 1: Go to wikimapia.org
STEP 2: Move your mouse around to find the map polygon you'd like to use, or use the search bar on the top right to look for your desired location.
STEP 3: Click on the layer you'd like to import, a panel should appear from the left side.
STEP 4: Copy the URL, that should end with the name of the location you're trying to import.
STEP 5;: Go on https://geojson-getter.herokuapp.com/ and paste the url in the input
STEP 6: View the Layer and download it
STEP 7: upload the GeoJson file onto your map in your project settings