You don't, it's not supported...
You need to partition first and then configure the unattend.txt file
to use the existing partitions. Very easy if you are using WinPE (and
diskpart) for deployment, but there are other tools that can be used.
regards
Johan Arwidmark
Microsoft MVP - Setup/Deployment
>I want Windows XP unattended installation to partition my hard drive to
>equal two parts. How do I make this?
>
>Thanks in advance
>Adam