If you have any older motherboard with Windows 10 (even ones that had older Windows and upgraded to 10), you might be able to transfer the license.
Try this first:
https://support.microsoft.com/en-us/help/20530/windows-10-reactivating-after-hardware-change
If that fails, and you have proof-of-purchase of any computer with Windows 10 Retail (meaning not OEM pre-installed), and proof-of-purchase for your new motherboard, you can try to go on Microsoft support and see if they'll transfer it.
I got a free license through the Insider Program (when those were available free, just before launch) and I've since transferred and continued using it.
You can also just not activate. There are no downsides to having a non-activated version of Windows, other than a watermark on the screen (it's a small translucent watermark in the bottom right that says "Go to Settings to activate Windows"), and not being able to change the accent colour (unless you change on a different PC and sync the setting). If you can live with the watermark, you can use Windows for free.
Also if you need it, the download for Windows 10 is here:
https://forums.windowscentral.com/e...Fsoftware-download%2Fwindows10&token=VVrmhQAS (this download works to install any version of Windows 10 on any computer, including OEM, no key required)