Whether activation occurs automatically will depend on what deployment method you're using and how your task sequence is configured. You can verify if Windows has been activated after installation by opening My Computer, clicking on System Properties, and scrolling down to the Windows Activation section.
If you're using the Microsoft Deployment Toolkit (MDT) there is a task sequence step for Activate Windows. You can also add a custom command via whatever method you prefer (through unattend.xml, a task sequence step, etc.) to activate Windows using slmgr.vbs:
slmgr.vbs /ipk <MultipleActivationKey>