I'm assuming that you're working on an M1 or M1 Mac since the tool is intended to download ARM versions of Windows (and the tool doesn't work on Intel Macs).
What you're seeing has nothing to do with BIOS settings. Fusion creates a virtual machine with UEFI firmware because a) Windows 11 will not support legacy BIOS and b) Fusion does not support legacy BIOS on M1/M1 Macs. There's no setting to switch a VM on M1/M1 Macs to Legacy BIOS nor can you edit the VMX file to do so.
Here's what your problem likely is:
When you boot ANY Windows ISO installer (Intel or ARM), the it displays a message:
"Press any key to boot from CD or DVD"
This prompt times out in about 5 seconds. You must respond to that message by clicking the mouse in the VM's console window (or type Command-G), then press any key on the keyboard. If you don't do this, the VM will attempt to boot over the network - and that will eventually time out.
Power off the VM and power it on again. Then reply to the boot prompt as noted above.