bad hard drive replaced need windows 8 install

I had to replace the hard drive in a Dell Inspiron 3520 but the only Windows 8 .iso I have tells me "The product key entered does not match any of the Windows images available for installation. Enter a different product key." I did not enter a product key in the first place and from my understanding the key is burned into the BIOS at the factory. I believe I need a Windows 8 install disc/file from Dell. How do I get one?