Examine source code of pthuser-agent-rewriter

{
  "description": "User agent string can be selected from a popup menu using the browser action icon on the toolbar.",
  "manifest_version": 2,
  "name": "pthuser-agent-rewriter",
  "version": "1.2",
  "homepage_url": "https://github.com/mdn/webextensions-examples/tree/master/user-agent-rewriter",
  "icons": {
    "48": "icons/person-48.png"
  },
  "permissions": [
    "webRequest",
    "webRequestBlocking",
    "https://httpbin.org/*"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_action": {
    "default_icon": "icons/person-32.png",
    "default_title": "Choose a user agent",
    "default_popup": "popup/choose_ua.html"
  }
}