How to add JavaScriptInterface in Flutter WebView for Android?

I’m writing a plugin for the flutter. I have native code with JSInteface on native code.
How I can use this on flutter? Can I somehow forward a web view instance to the native code? Or My interface to the flutter so I can attach in some way? Code is open-source on https://github.com/Webtrekk/webtrekk-android-sdk-v5 class name with JS interface is WebtrekkWebInterface.
Thanks in advance