Microsoft Windows 8 Serial Key Original 'LINK'

Could you please help me how could i find my windows Product Key. My original hard disk got corrupted and is not responding when connect to laptop, so i had to purchase a new hard drive and i did not make a note of the windows product key. Now once i instal windows 10 in new machine it is asking for product key could you please tell me how do i get it from dell service tag. This was a windows 8 laptop when i bought it

Microsoft Windows 8 Serial Key Original