IIS - External Windows authentication issue

[googlemad]

We have a osTicket support desk system which users can either access internally or externally.

 

This is set up as a website within IIS with the following authentication :

 

Anonymous authentication : Disabled

Windows authentication : Enabled HTTP 401 Challenge

 

Internally it works fine, users can access and their Windows credentials are passed through to authenticate

 

Externally users get a login box so that they can enter their Windows credentials. This is fine APART from on some user machines the login box assumes they want to login to HOMEPC\account rather than OURDOMAIN\account!

 

Is there any way to force the login to map to OURDOMAIN by default? I did try enabling the Anonymous authentication but this then broke the SSO for osTicket!