Windows 8 product key, Dell Inspiron 14z

Hello,

3 year ago I bought a new Dell Inspiron 14z with windows 8. I have several pratitions on it and I dual boot with Kubuntu. Later I upgraded to windows 8.1 and 10.

Now  I just saved some programs and all personal files on a external HDD. I formated my C drive and reinstalled with an USB windows 10. Now I have to enter a product key which I am not able to find under the computer. I tried to run in cmd.exe, Slui 4. I followed the instruction and Microsoft called me, I gave them my installation ID. They just said they can do nothing I even didn't gave me any advice.

I also looked at the receipt, there were a lot of numbers but nothing like a win8 product key.

I tried to look into the PC but I can just open at one place where I think the SSD is located.

Under the laptop I have a Dell sticker with some data and numbers but no win8 product key and a windows 8 sticker with the windows logo.

What can I do now?? Is there a possibility something is some hidden partition that I am able to see in Kubuntu.

( Kubuntu worked after reinstall but now I made some changes and it doesn't work anymore bit I think I'll get it to work)

I hope someone can help me.

Mike