How can I define the path in the browser?

I would like to load the browser with a defined path - currently it always starts with http://localhost:3000/, but I would like to start it with http://localhost:3000/subfolder/
I can easily set the root path of the server. But my app structure requires that the root path is different from the path to my index.html. I would like to set my root path to './' but the browser should open /app/index.html.

Thanks,
Geri