Windows 7 Credential Manager and ntlm proxy authentication untrusted domain

Hello experts

I work in an organisation where we have our own local AD and have our upstream internet provided through another section of the organisation complete seperate to us with a totally separate AD with no trust between the two and there will never be one implemented.

Our local AD username and passwords generally match the upstream ones in our parent companies AD environment but we have different domain names. Our parent company has an authenticated proxy whereby windows credential manager has been happily working and supressing the need for the end users to enter logon credentials to the web browsers when we open them until our parent company changed to an NTLM proxy.

Since they implemented the NTLM proxy all users are now prompted to logon and have to remember to put @otherdomain on the end and even though windows credential managers saves the details it prompts every time we open the browser still so its pointless it offering to save the credentials. I'm hoping there is a way to fix it so we suppress the proxy logon prompts again using credential manager or some other fix in windows. Is there a way anyone can suggest for me to somehow automatically submit a username with @otherdomain for the proxy logon for the web browsers ?