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

[Nick Howitt]

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