This is actually in the FAQ, so you don’t need to contact support – you can simply buy the new license for the Windows edition (or go to the Windows beta forum for the beta download link and use that until version3 is released):
https://www.literatureandlatte.com/scrivener/faqs#release1
And, as xiamenese notes, you can get the multi-platform discount, so you’re not even paying full price to buy the Windows license – plus, when you buy it now, it will give you the upgrade to version 3.x for free once that releases.
Note that If you have Scrivener v3 on your Mac and you want to seamlessly transfer your projects to your Windows device, you should probably check out the beta. If you go with the current release 1.9.x version of Scrivener for Windows, you will first want to use the “Export to v2” feature in Mac Scrivener to save your projects into the v2 format so that Scrivener for Windows 1.9 can recognize the projects.