Examine source code of Save-Data: on

{
  "manifest_version": 2,
  "version": "1.1.0",
  "author": "Daniel Aleksandersen",
  "background": {
    "persistent": true,
    "scripts": [
      "background.js"
    ]
  },
  "description": " Automatically enable data-savings mode at compatible websites.",
  "icons": {
    "16": "img/icon16.png",
    "48": "img/icon48.png",
    "96": "img/icon96.png",
    "128": "img/icon128.png"
  },
  "name": "Save-Data: on",
  "short_name": "Save-Data",
  "permissions": [
    "<all_urls>",
    "webRequest",
    "webRequestBlocking"
  ],
  "incognito": "spanning",
  "offline_enabled": true,
  "homepage_url": "https://www.daniel.priv.no/web-extensions/save-data.html"
}