Examine source code of Link Optimizer +

{
  "manifest_version": 2,
  "name": "Link Optimizer +",
  "version": "1.0",
  "description": "Create short links and QR codes with Ulvis.net",
  "icons": {
    "16": "extension_icon@16px.png",
    "32": "extension_icon@32px.png",
    "48": "extension_icon@48px.png",
    "128": "extension_icon@128px.png"
  },
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": {
      "16": "extension_icon@16px.png",
      "32": "extension_icon@32px.png",
      "48": "extension_icon@48px.png",
      "128": "extension_icon@128px.png"
    }
  },
  "permissions": [
    "tabs",
    "clipboardRead",
    "clipboardWrite",
    "https://spoo.me/*"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{ff520f2a-ee26-4e8f-b9ef-589482148e23}"
    }
  }
}