It will depend on what licensing method you are using. This
Licensing and Volume Activation page from the Springboard Series on TechNet has a ton of articles on activating KMS and MAK (both of the volume key types).
This article shows what unattend setting to use to input the product key (Microsoft-Windows-Setup | UserData |ProductKey). Finally,
this thread discusses using scripting for activation and to remove the unattend file after deployment so that your product key is not left on the deployed system in unencrypted form. More on the
RunSynchronous and
RunAsynchronous commands to run scripts during deployment.
Hope this helps,
David
Windows Outreach Team - IT Pro