I'm 99% sure you can still get a free Windows key by upgrading from 7 to 10. I have done it 3 times now, although about a year ago ago, but still far after the free upgrade expired.
Just use the media creation tool from MS and upgrade through that to do it.
Just make a copy of your window 7 disk and then when upgrading it should work out and if not you can always roll back.
I believe they stopped that. But you can buy keys for around £20 which is still a good deal to me. Once you registered last year, it was ok. I don't think it works for new users though.
Guru3D had a link not so long ago promoting it.