Windows 10 product key is embedded in the motherboard. 

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Thanks for your reply.

For your information, motherboard is burnt and I cannot get warranty. I want to install the OS with a different motherboard and don't want to bear additional amount for OS license. How can I do that?

If COA sticker is destroyed? on machine, there ought to be a sticker residue to show where it was.  look around it more carefully for it in case you missed it the first time.  also look inside all paper documents that came w original purchase.  It still not found, options are using Win 7 product key from some older pc, or installing Win 10 without key for now until a Win 7 key becomes available in the future.  any genuine win 7 key is good for activating Win 10 as long as the key has not been used to do so and win7pro key for win10 pro or 7home for 10home.

@EhsanBD 

Which model Optiplex?

@nyc10036 

Optiplex 3050

Windows 8 and 10 systems have key in system SLP key on the board.

You cannot transfer the key from one board to another.

If its burnt and still under warranty you should be able to get replacement.

Thanks for your answer. I'll try to do so.

You need a new motherboard and make sure you don't burn it!

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@EhsanBD 

Do what?

You wrote that you cannot get warranty.

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@nyc10036 

The service provider told me, as this is burnt I'm not eligible to get in warranty repair.