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
That one is ignored by Windows Setup, but doesn't hurt to set it there. The other is further down
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.
Deployment Server 6.9 SP1 and prior releases