It looks like windows authentication enabled. I don't think you can fill the username there by default, unless that is an asp form/page. But on cancel of that window, it shows up the page where it asks for username/password, is that the same that was asked in the popup earlier? If so you can try posting username to that page and request the same within there and show it up as default. not sure if you have control over making any changes in that page.
- Vijay G
Strive for Perfection