Activation code? Probably mean Activation KEY for Windows 8.1?  Windows 8.1 is an update and shouldn't be the OS pre installed, or it would be on the sticker on the Laptop, or it would be in the documentation that came with it when you purchased it.  Did you get any of those golden priceless pieces of paper and plastic with it?   if you got it 2nd hand (from a private owner) then it was probably pre-installed with 8.0 and then upgraded to 8.1.  If so; you have to install 8.0 FIRST and ACTIVATE it .. before running the upgrade to 8.1. I'm trying to find a windows 8.0 site for download but only finding 8.1 ...
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?
Activate Windows: Running Windows as a virtual machine on Ubuntu: How to 
activate?