Using IIS URL Rewrite, how to rewrite foo.bar.com -> bar.com/myapp

Question

Our web app lies at bar.com/myapp

We'll use the HTTP Host Header to work out the username

So need to transparently rewrite http://foo.bar.com to http://bar.com/myapp using the URL Rewrite module in IIS

But still need to be able to go to www.bar.com and see the company website and webmail.bar.com, etc.