Redirect default website of Exchange 2010 to OWA login page

Trying to figure this one out.  I am using TMG to publish OWA to users on web.

I want users to be able to enter https://webmail.mydomain.com and got to the OWA signon page.

However - it seems that this does not work - internally this just brings me to the IIS7 page of the Exchange 2010 server.

If I enter https://webmail.mydomain.com/owa then I get the login page to signon HTML form.

And related to this - which I think is another post - I try to login and I go nowhere - I enter my Creds but get no joy.

Thanks

J