Access to Exchange 2010 OWA

We have a working Exchange 2010 environment. We wan to insert the F5 to do the load balancing of the CAS servers. We followed the design guide and for the most part the F5 is doing its job. However, if we enter invalid credentials (or simply click the logon button without even entering credentials, the F5 presents us with the Outlook Web App logon screen. I think this means that I need to go into the access policy and use VPE to edit the policy. The logic seems confusing to me so I'm hoping someone can tell me what I need to edit to deal with this. I simply need to pass the username and password (no need for F5 to do any authentication) to OWA and deal with the situation where the user enters invalid credentials. Thank you in advance.