Examine source code of Watch Later button

{
  "manifest_version": 3,
  "browser_specific_settings": {
    "gecko": {
      "id": "pletnov@duck.com",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "name": "__MSG_extensionName__",
  "description": "__MSG_extensionDescription__",
  "default_locale": "en",
  "author": "Alex Pletnov",
  "version": "1.5.1.1",
  "icons": {
    "16": "assets/app-icons/red/iconmonstr-text-plus-lined-16.png",
    "32": "assets/app-icons/red/iconmonstr-text-plus-lined-32.png",
    "48": "assets/app-icons/red/iconmonstr-text-plus-lined-48.png",
    "96": "assets/app-icons/red/iconmonstr-text-plus-lined-128.png"
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "action": {
    "default_popup": "popup/popup.html"
  },
  "content_scripts": [
    {
      "css": [
        "assets/styles/style.css"
      ],
      "matches": [
        "https://www.youtube.com/*"
      ],
      "js": [
        "js/saveto.js"
      ],
      "run_at": "document_start"
    }
  ],
  "permissions": [
    "scripting"
  ],
  "host_permissions": [
    "https://www.youtube.com/*"
  ]
}