Examine source code of SteamAutoLink

{
  "manifest_version": 2,
  "name": "SteamAutoLink",
  "version": "2.1resigned1",
  "description": "Adds button that opens Steam pages into your Steam application.",
  "icons": {
    "48": "icons/icon-48.png"
  },
  "permissions": [
    "storage"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "web_accessible_resources": [
    "images/logo.png"
  ],
  "applications": {
    "gecko": {
      "id": "steamautolink@lonedev.ml"
    }
  },
  "content_scripts": [
    {
      "matches": [
        "*://store.steampowered.com/app/*"
      ],
      "js": [
        "openapp.js"
      ]
    },
    {
      "matches": [
        "*://steamcommunity.com/"
      ],
      "js": [
        "openCommunityHome.js"
      ]
    },
    {
      "matches": [
        "*://steamcommunity.com/app/*"
      ],
      "js": [
        "openCommunity.js"
      ]
    }
  ]
}