Load Drivers, Windows 2019 example
Start with a normal Virtual Machine Creation using the Build tab.
Do not forget to select a win* in OS list to automatically mount the VirtIO ISO.
Once the server created, go on console and follow the standard Windows installation process.
When you arrive to the disk formatting part, you will not be able to select a disk as Windows don't have the drivers to recognize them.
For this, select Load Drivers.
Now, Browse to find the driver CD in the list and select the folder according to the Windows version you want to install.
Once loaded, you will be able to see the disks and continue a standard Windows Install.