Projects like OPNsense.org provide you with an .img file that you would dd to a USB device to boot from. This is not obvious how to use from VirtualBox. You need to convert that into a VMDK file. Basically, the command I used was:
vboxmanage convertfromraw OPNsense-19.7-OpenSSL-serial-amd64.img /tank/VMs/4544-opnsense-19-freebsd/opensense-19.7-usb.vmdk –format vmdk
Then attach that VMDK file to your virtual SATA controller and when you boot really quick! Hit F12 and choose option 2. That’s your USB device.