I don't think you can - this would be an opportunity for a security leak if it were possible - you'd be able to get the name of the computer and then via WSDL tell someone else the name of the computer the user is using.
You can get the platform (xfa.host.platform) and the host application (xfa.host.name, usually "Acrobat") but nothing that's particularly identifying.
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SteveX
Adobe Systems