Launch Parallels Desktop.app.
Plug in the Windows 10 installation USB stick to your Mac.
On Mac menu bar locate File > New.
Choose Install Windows or another OS from a DVD or image file.
Windows 10 image should appear in the list, referring to USB stick.
Either type in Windows 10 license key before installation or uncheck This version requires a product key option to activate Windows later.
Choose what activity you use Windows 10 for.
If you wish to change the initial name of your virtual machine, location or even preconfigure the settings, check Customize settings before installation option and click Continue.
Otherwise leave it as this and click Continue.
At this point Windows 10 installation should start and after a few moments you should see Windows Setup wizard.
Choose Windows 10 edition you purchased and click Next.
Wait for the installation to finish.