What do you mean by "product key"? DIY motherboards don't come with a Windows key or license, you buy that separately. Only pre-built systems from large PC manufacturers come with a Windows product key stored on the motherboard.
If this is a used system that you just bought, and it was supposed to come with a Windows license, contact your seller for the Windows product key.
my computer is built by myself, but in armoury crate i can register the product. i want to register the product. i was told the would be a sticker on the back of the motherboard with the product key but it aint there
Ahh, ok. The product registration just uses the serial number, as far as I know. It should be on a sticker on the board, but the location may vary. On my board, for example, the serial number sticker is on top of the SATA headers, on the front of the board.