Try the SDK demo here.

Make sure you have standalone Git installed first. Reboot after installation.
In Unity, open "Window" -> "Package Manager".
Click the "+" sign on top left corner -> "Add package from git URL..."
Paste this: https://github.com/forcepusher/com.agava.yandexgames.git#16.0.0
See minimum required Unity version in the package.json file.
Find "Samples" in the package window and click the "Import" button. Use it as a guide.
To update the package, simply add it again while using a different version tag.

This package automatically inserts YandexGames SDK script into HTML page when calling YandexGamesSdk.Initialize(). No need to mess with WebGLTemplates - just use the SDK methods.

This is a publishing repo. If you need to create a pull request, use the Development Repo.