Examine source code of Youtube Play All

{
  "manifest_version": 2,
  "name": "Youtube Play All",
  "description": "Adds a 'Play All' button to YouTube, allowing video playback in Latest, Popular, or Oldest order.",
  "version": "0.4.2",
  "content_scripts": [
    {
      "matches": [
        "https://m.youtube.com/*",
        "https://www.youtube.com/*"
      ],
      "run_at": "document_end",
      "css": [
        "content-scripts/play-all-button.css"
      ],
      "js": [
        "content-scripts/play-all-button.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{f082384d-0ec3-4a0a-a2d1-3261bb1b8d8e}"
    }
  }
}