Where find Windows 8 key of my XPS 8500 in order to reinstalled it

Hi,

I have bought a new dell XPS 8500 and came with Windows 8 preinstalled.

But in other to get a save copy of Windows 8 from Microsoft, i need to find that key

(Habitually, desktop have a sticker with the key wrote on it,but the one I got have any on its surface...)

Is that key is saved somewhere on the computer or either i need to disassemble the desktop to search inside ?

Thank you