In this post, I am giving the powershell commands to configure OWA authentication mechanism as per your needs usign powershell cmdlets. Please write to me (in comments sections) if you need any clarifications.
Configure Exchange 2007 OWA to take default domain name:
Set-owavirtualdirectory -identity "owa (default web site)" -LogonFormat UserName -DefaultDomain mydomain.com
Configure Exchange 2007 OWA to accept only “domain nameUser Name” format:
Set-owavirtualdirectory -identity "owa (default web site)" -LogonFormat fulldomain
Configure Exchange 2007 OWA to use form-based authentication:
Set-owavirtualdirectory -identity "owa (default web site)" -FormsAuthentication:$true
Configure Exchange 2007 OWA to use integrated authentication:
Set-OwaVirtualDirectory -Identity "owa (Default Web Site)" -WindowsAuthentication $true
Happy Learning,
Sitaram Pamarthi