APM logon page with domain

Rookie question I'm sure, but I'm failing to understand how to capture the domain of a user logging in on a basic logon page.

I have been successful at getting AD to authenticate with just the basic username with password because it is setup to default to a domain, but the Exchange server needs the domain to be provided. I've been told that I should have the F5 APM logon screen accept a format of 'domain\firstname.lastname' or 'firstname.lastname@domain'. I know how to make another input field to specify a domain, but that feels clunky. I was under the belief that most logon pages just ask for the user to provide all of the logon information in one field, and I'm trying to replicate that.

This doesn't seem difficult, but I'm not successful thus far. Any insight would be appreciated.

Note: I was successful with a test of simply adding a Variable Assign for a static domain and passing that through, but I've told that there are multiple domains so this wouldn't work. :(

Thanks, -Warren