Windows 10 Product Key how to get ?

Hello everyone here,

So I'm having Dell Inspiron 15 3543 Model and it was running on Windows 10 but recently the hard disk got corrupted due to some unknown reasons so i have to put new hard disk and i i told a friend of mine to do this work but he has installed pirated windows 10 pro with some random product key on internet and due to this I'm not getting any security updates so i i wanted to know how to get the 25 characters product key which i got originally with laptop (I've searched on box as well as on laptop there is no sticker regarding product key) and how should i i install genuine windows 10 and what should be my bios settings while booting the windows 10....

Thanks in advance....