Examine source code of Medium Popup Blocker!

{
  "description": "Blocks medium.com popup asking you to sign up",
  "manifest_version": 2,
  "name": "Medium Popup Blocker!",
  "version": "2.0",
  "author": "Macko Martin",
  "homepage_url": "https://github.com/LinkedList/medium-popup-blocker",
  "icons": {
    "48": "block.svg"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.medium.com/*"
      ],
      "js": [
        "block.js"
      ]
    }
  ]
}