BASIC Auth on APM for web server

I am trying to put authentication in from of a IIS web server that is not running any kind of authentication.

I can get it to work successfully by using a logon page ie -

However as this is going to be called by a web service a logon page is not an option. Instead I am looking to present a 401 response using BASIC authentication. See below -

The problem I am having is that while it is authenticated and there is a session created the F5 is then trying to present the credentials entered in the 401 challenge to the web server in the default pool.

I don't want to present the credentials to the web server as the whole problem I am trying to solve is the fact that the web server is not running any authentication.