The appRootUrl setting does not actually effect how/where the runtime actually is accessed. The setting exists to tell a Mendix app where it should expect inbound traffic to come from. This influences the “view” button Studio Pro as well as the ability for the Mendix runtime to build a URL to itself. The runtime needs to know its own URL in order to:
In order to actually make your app accessible at a particular URL, you usually would use a web server. The web server often serves up static content itself, forwards all dynamic data requests to the Mendix runtime, and (optionally) adds SSL/TLS. On Windows that means configuring IIS, and on Linux typically you’d use nginx. With either, you’re creating a reverse proxy from a URL accessible outside the server to the jetty-based Mendix runtime server running (usually) at localhost:8080. In your case it looks like it’s running on 8081.
Setting up the web server for Windows is discussed here, and for Linux here.