See example of using iframe and the WBI login page by viewing http://www.ayanova.com/iframe-example.html
View the page source to see the HTML used (i.e. if using Firefox, just right-click on the web page and select View Page Source)
Please note that your WBI does need to be updated to the latest WBI 7.2.0.0 Hotfix 4 to ensure WBI expands to full page when log in as a client or head office.