Custom user agent string available in Metro IE but not IE 10 (desktop) on Windows 8

I've added a custom user agent platform to the registry HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\5.0\User Agent\Post Platform. The name appears in Metro IE navigator.userAgent, but not IE 10 (desktop mode). I've restarted Windows 8 without any change.

Any ideas on how to change the IE 10 user agent string? (Using the IE developer tools isNOT a viable solution).

Examples:

IE 10: Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; WOW64; Trident/6.0; .NET4.0E; .NET4.0C; InfoPath.3)

Metro IE: Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; Win64; x64; Trident/6.0; .NET4.0E; .NET4.0C; InfoPath.3; MyCustom)

Notice that Metro IE includes "MyCustom".

keywords: IE10 Internet Explorer 10 Win8