So you are really after the static page, not the API

You have to register the route for your page by overriding WidgetRoute() class. So when your Flutter app calls the url, the registered route in server.dart will call the relevant WidgetRoute(). This will return the Widget() which can be your HTML page

The HTML page will be fetched from the web/static/templates folder on the server

This is now in the serverpod documentation