Make a copy of the Longhorn_unattend.xml file, and replace the %OS_PRODUCT_KEY% token with the product key. There are two locations to change it, one is just under the Computer Name
<!--ProductKey>%OS_PRODUCT_KEY%</ProductKey-->
That one is ignored by Windows Setup, but doesn't hurt to set it there. The other is further down
<Key>%OS_PRODUCT_KEY%</Key>
That one needs to be set. If desired the value can be left blank ( <Key></Key> ) and Windows Setup will prompt for the product key.
Be sure to remove the entire token including the %'s otherwise Windows Setup will not be able to parse the unattend answer file.
Applies To
Deployment Server 6.9 SP1 and prior releases