How to get original 'User-Agent' header

We're trying to get the original 'User-Agent' and client IP Address from our backend IIS logs, but we're seeing only the IP of the F5, and generic USer-Agent entries.

I understand I can get the original client IP by allowing 'X-Forwaarded-For' headers to be inserted, but how about the original 'User-Agent' heaaders ?

Thanks !!