You need to use Dell OEM System Locked Preinstallation. More details about the activation mechanism here: [URL]  Since you purchased your system with Downgrade Rights - the Dell Skylake .iso should be Downloadable see here: [URL] 
I purchased a new Dell Optiplex 5040 that came with Win7 Pro 64 installed and included a licensed Win 10 Upgrade DVD.  I Just installed  a fresh Win7 Pro 64 onto a new SSD and found out that there is no Win 7 product key sticker on the machine like Dell typically includes. I used 3rd party software to extract the windows product key from the original HDD and used that when booting up the SSD but windows gives an error saying that product key is invalid. How do I reactivate an OEM Dell Win 7 if there is no key sticker on the machine? Thank you in advance for any help you can offer.  Gary 
Win 7 Pro64 Dell Factory downgrade reactivation