Examine source code of WhatsApp Web Button

{
  "manifest_version": 2,
  "name": "WhatsApp Web Button",
  "version": "0.0.5",
  "description": "Add a WhatsApp Web button to any telephone number",
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "script.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{da1ab602-d92f-4b1a-b42b-b17efde2ed7d}"
    }
  }
}