Set OWA Default Domain Using Basic Authentication

Is there a way to set the default domain so that Exchange 2010 OWA users can just enter their user name and password instead of domain\username if Basic Authentication is being used?

Exchange 2010 OWA published through ForeFront TMG 2010 with Basic Authentication.

Exchange 2010 mgt console allows the default domain to be set if Forms Based Authentication is used, but we can't figure out how to set that for Basic Authentication.