In Webview, share button not working What I do?

You can also use deep-link

https://docs.thunkable.com/thunkable-cross-platform/2-create/blocks/control#open-an-installed-app-by-deep-link

deeplink

then in this case, how do I link the button of my site to share in this “whatsapp: //”

This is impossible in a simple way. Currently there is no data exchange between the Web and the native part of the application.

The difficult way is to organize such a connection using Firebase.

The nuance is for each Firebase user to have his own separate data node for exchange.