Examine source code of Custom Function Runner

{
  "manifest_version": 3,
  "name": "Custom Function Runner",
  "version": "1.0",
  "description": "Create and run custom functions on any webpage.",
  "permissions": [
    "storage",
    "activeTab"
  ],
  "action": {
    "default_icon": "icons/icon-48.svg",
    "default_title": "Custom Function Runner",
    "default_popup": "popup/popup.html"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content_scripts/runner.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "custom-runner@example.com"
    }
  }
}