What is the magic to see the Windows User as DOMAIN\user

I'm sorry for posting this here as it is really a Windows question but it is driving me scatty trying to authenticate with a Windows client. I only seem to get the User-Name come through with machine credentials like host/machine_name.DOMAIN. For the life of me I can't see how to switch Win10 to pass a real user name through to Radius like DOMAIN\user. There must be a setting in Windows which I am overlooking. Can anyone please help? Thanks, Nick