Examine source code of YT Captures+

{
  "manifest_version": 3,
  "name": "YT Capture+",
  "version": "1.0",
  "description": "Take screenshot of YouTube video only.",
  "permissions": [
    "scripting",
    "activeTab"
  ],
  "content_scripts": [
    {
      "matches": [
        "*://www.youtube.com/*"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "action": {
    "default_icon": "icons/ext.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "ytcapture@example.com"
    }
  }
}