Examine source code of InstalingPlus

{
  "manifest_version": 3,
  "name": "Paste Ext Instaling",
  "version": "1.1",
  "description": "Allows pasting text into input fields.",
  "permissions": [
    "activeTab"
  ],
  "content_scripts": [
    {
      "matches": [
        "https://instaling.pl/ling2/html_app/app.php?child_id=*"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "icons": {
    "16": "instaling-logo.png",
    "48": "instaling-logo.png",
    "128": "instaling-logo.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "ItsAlex@ItsAlex"
    }
  }
}