You are probably aware that this is no ordinary request, right? You can only
do this if your client can 'see' a domain account. Typically, your client
hosted in the browser has no direct access to the server so if the domain
account is on the server you won't be able to do this easily. If your code
running your client has access to a user context, you can extract the user
from the context and turn around and fire a active directory query but in
most cases the user context is null. The cheap way to see if your existing
code works is simply to turn cas policy off at the command prompt and see if
the application works. If it does, then you simply need to configure CAS
policy.

One way is to have an app or web service running on the server that can do
what you want. From your client, you just fire a web request to the server
to 'authenticate' the client.