You can not specify the domain in the Portal Gateway because it doesn't allow special characters. You have to specify it in Internet Information Services (IIS). The following directions will walk you through specifying the domain.

Click the Start button
Right click My Computer then choose Manage
Click the plus sign (+) next to Services and Application
Click the plus sign (+) next to Internet Information Services
Click the plus sign (+) next to Web Sites
Click the plus sign (+) next to Default Web Site
Click on OSI
Right click on OSI Portal Services and choose Properties
Click on the tab labeled Directory Security
Click the Edit button under Anonymous Access and Authentication Control
Make sure Basic Authentication is checked and specify the domain here