You don't need a key with the Lenovo machines. You can use the Media Creation Tool from MSFT to create an installation media for your version of windows and it should just install.
I wiped my Lenovo's entire SSD then put a fresh copy of Win8 and then upgraded to Win10 no problems.
Yup, I've had good luck with the MCT although the new one for 7 might require a key in the first place. I haven't tried it because I have all of the Digitalriver ISOs archived but perhaps you could try the OEM install key (pull it using a freeware called
Produkey). I bet you if you Google that key, you will find hits relating to Lenovo because this is not what activates an OEM license, it's a certificate stored under %windir%\system32\oem\ . Before you reformat, just copy everything from that folder to plop in the new installation, choose to skip the key entry during the clean installation and run these two commands in Admin CMD post-install :
cscript %windir%\system32\slmgr.vbs -ilc %windir%\system32\oem\OEM.xrm-ms
cscript %windir%\system32\slmgr.vbs -ipk [full key here, dashes and all]
It's worth it.