Insert HTML by Designmodo

manifest.json

{
  "update_url": "https://clients2.google.com/service/update2/crx",
  "manifest_version": 3,
  "name": "Insert HTML by Designmodo",
  "short_name": "Insert HTML in Gmail",
  "description": "Insert custom HTML format Gmail.",
  "version": "1.0.4",
  "host_permissions": [
    "*://mail.google.com/*",
    "*://inbox.google.com/*"
  ],
  "content_security_policy": {
    "extension_page": "script-src 'self' 'unsafe-eval'; object-src 'self'"
  },
  "content_scripts": [
    {
      "matches": [
        "*://mail.google.com/*",
        "*://inbox.google.com/*"
      ],
      "js": [
        "js/inboxsdk.js",
        "js/injectInCompose.js",
        "js/injectInSign.js",
        "js/contentHelper.js",
        "js/content.js"
      ],
      "css": [
        "css/content.css"
      ]
    }
  ],
  "icons": {
    "16": "icons/16.png",
    "48": "icons/48.png",
    "128": "icons/128.png"
  },
  "web_accessible_resources": [
    {
      "resources": [
        "resources/*"
      ],
      "matches": [
        "*://mail.google.com/*",
        "*://inbox.google.com/*"
      ]
    }
  ]
}