If their site thinks you're not on Windows, they'll offer you direct download links to the ISOs valid for 24h. Before installing Windows 10 in VirtualBox, lets see what you need first: An internet connection with good speed, or a Windows 10 ISO that youve already. The reason is that if they see you use Windows they'll offer you their downloader EXE, which can only download to drive C: and if it's full (which is usual when using a small SSD drive as the main one) you can't download even if you have a secondary drive with several free TBs, since you can't choose another drive.
I used this one for Chrome: (User-Agent Switcher for Chrome) You just have to either use a non-Windows PC (Linux, Mac, Android, whatever.) or use a user-agent spoofer tool so Microsoft's website thinks you're not in Windows. Then, follow the on-screen to start the installation. Disable audio by unchecking the checkbox. Click the little CD icon (right) and select your ISO file attach ISO file to the virtual cd drive.
After choosing a language and keyboard format, click Install now to continue. Select your chosen ISO file as the optical drive. Then, boot the PC from that drive for a clean install. There's another option to download the ISOs from Microsoft. Next, download Rufus and burn the Windows Server 2012 R2 ISO file you have downloaded to a USB flash drive to get a bootable drive.