How to redirect to folder via webconfig?

Ok so my root folder looks like this:
- aspnet_client
- index.html
- Webconfig
- SUB (FOLDER)

I have pointed my sub domain name A Record to ftp root folder and in webconfig, I am pointed the request from "sub.sitename"  to folder "Sub"
When I enter "www.sub.mysite.com", it shows the content in "Sub" folder without any isssue. BUT - when I enter "sub.mysite.com", it "page not found / server not fount / enter address correctly " please help?- I want it for work without "www" also becasue a lot of time users don't type "www" while going to a sub domain site.

Here is my webconfig file:
<?xml version="1.0" encoding="UTF-8"? ...