Activate Windows: Running Windows as a virtual machine on Ubuntu: How to activate?

I just got a Latitude 3550 with Windows 8.1 preinstalled. I assume they were OEM activated when I first switched on the machine. Then I installed Ubuntu 14.04, completely formatting the disk (there are no ntfs partitions anymore, so Windows are gone).

But since I paid for the Windows, I would have liked to run them using a virtual machine. Trying an installation using the DVD that came with the laptop using VirtualBox, I am asked for an activation code. 

Do you know if it is possible to get one since I am not using the original preinstalled windows anymore?