Open VirtualBox Select the guest OS, but don't launch it. Click on the Settings button. Go to System > Motherboard > Boot Order Select an entry (e.g Hard Disk, USB, or DVD drive) and use the arrow ...

I don't think Virtualbox has a UEFI option, so I think virtualbox is (rightly) booting legacy. But still it can't boot from that ISO I just tried VMWare Player I notice that one can choose UEFI or not ...