Examine source code of Focus Blocker helps

{
  "manifest_version": 2,
  "name": "Focus Blocker",
  "version": "1.0",
  "description": "Limit time on distracting websites like Facebook and YouTube.",
  "permissions": [
    "storage",
    "tabs",
    "<all_urls>"
  ],
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "browser_action": {
    "default_title": "Focus Blocker",
    "default_popup": "popup.html"
  },
  "icons": {
    "48": "icon.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{82b904bd-c131-45b3-8b55-520518c99bf6}"
    }
  }
}