DEar sko :The ancient CSM-firmware for bhyve has been deprecated over a year ago: sysutils/uefi-edk2-bhyve-csm (no, it won't build anymore)
IIRC one could modify a Windows 7 install medium to actually support UEFI - but you might be better off searching/asking for this in some windows forums...
but seriously: just dump that old crap
(or better yet that whole family of advertising/data-collection plattform still sold as an operating system...)