Examine source code of Medium Premium Reader

{
  "manifest_version": 3,
  "name": "Medium Premium Reader",
  "version": "1.2",
  "description": "Adds a button to Medium articles to transform the URL.",
  "permissions": [
    "activeTab"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "medium-transformer@yourdomain.com",
      "strict_min_version": "140.0",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.medium.com/*"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_end"
    }
  ],
  "icons": {
    "48": "icon.png"
  }
}