Linux

You can download the windows iso from windows webpage, but if I remember correctly you have to do it in Windows since it depends on a downloader program. Then you should store the windows cd-key just in case, you shouldn't need it and there are ways to recover it from Linux if this is an OEM machine, but it's always better safe than sorry.

With all that being said, a bit of unpopular opinion now. Why do you want to do that? I kept a windows partition for years because I never knew if I would need it, it was only when I realised I hadn't touched Windows in months that I felt comfortable removing it, and at that time I didn't cared if it could be reinstalled. The reason I'm asking is because if you have to ask about it you probably never installed windows on a machine, so you don't know how to do that, and it's a lot more complex than Linux because Windows needs all of the drivers to be manually installed to work properly, and while Windows 10 is a bit better in finding those drivers automatically it's still a Pain in the ass to ensure it got the correct things and wait the thousands of reboots to apply all updates because updates are cumulative in Windows.