Supplying Basic Authentication credentials to a protected folder in ASP.NET

Question

I have a Barclays payment system which posts its results back to an ASP.NET page inside a folder protected with Basic Authentication (the rest of the site uses Forms). The payment system works fine but I also have a credit system which means that if credit > total then the payment system is bypassed and I just want to call the page myself.

How to I supply a Response.Redirect or Server.Transfer with Basic Authentication details (i.e. username / password)?

Thanks for any help!