In IIS, under the web site  properties, click on the Directory Security tab and then on the Authentication and Access control button. If the Basic Authentication tab is checked, then a Default Domain textbox is enabled. There you can specify networkname.

Keep this in mind, however. This is a less secure approach because the password is sent over the wires in clear text. If access to the portal will be from outside of the firewall, I’d recommend that this approach be taken only in conjunction with installing an SSL certificate on the server and requiring SSL-only access from the outside. The SSL will handle the encryption of the password.

If all of your users are in the same domain, then you can set the default domian in IIS.

IIRC, the default domain name setting in IIS is only for basic
authentication.

The issue is informing the server where it should forward authentication
requests. By default, it will attempt to authenticate to the local SAM.
With IIS using host headers, it uses the web site  name as the server
name which generally is not correct even if you were using local
accounts.

I teach users to always enter the logon as domain\user just to avoid
having to enter it more than once to correct the errors.