URL rewrite to subfolder that does not exist

I am using IIS 7.5 and URL Rewrite tool as a proxy server.

external user enters this URL: http://www.mydomain.com/test99/some.dll/99/login.asp

the actual URL of the backend server is http://www.mydomain.com/some.dll/99/login.asp

the test99 subfolder does not exist on the backend server, it is only present in the external URL.

how can i set this up so that "test99" stays present in the external URL, but still browses correctly on the backend server?

Thanks for any responses.