https://gist.github.com/nora-tetsu/6f5f0afd1b44f281b824979b409d010e と同じフォルダに入れてChrome拡張機能として読み込む

Dynalist_TextCounter_manifest.json

{
    "name": "Text Counter of Dynalist",
    "version": "1.0",
    "manifest_version": 3,
    "content_scripts": [
        {
            "matches": [
                "https://dynalist.io/d/*"
            ],
            "js": [
                "counter.js"
            ]
        }
    ]
}