You can't boot from a MBR because this kind of boot requires putting your computer in Legacy boot mode and Windows 10 does not boot in legacy mode.
If you want to double-boot Windows 10 and FreeBSD, then your boot mode needs to be UEFI.
Preparing the disk: It does not make any difference if you put the boot loader on a USB disk or a hard drive. Both are block devices and are treated the same, provided your computer's BIOS can boot from USB.
Preparing a UEFI device is described in the Wiki:
https://wiki.freebsd.org/UEFI
Just make sure, doublecheck and triplecheck that you are working on the correct device:
gpart show
- make sure the device you are working on is the USB disk (for example, da1).
Bash:
# You need to create a GPT partition table on da1:
gpart create -s gpt da1
# Then create an EFI system partition:
gpart add -t efi -s 256M da1 # It needs to be formatted with FAT16, so do not use the whole USB drive (it's probably too large for FAT16).
# No need to create a root partition (you already have it on your hard drive).
newfs_msdos -F 16 -c 1 /dev/da1p1 # I am not sure but FAT32 might also work with EFI, check this to be sure
mount -t msdosfs /dev/da1p1 /mnt
mkdir -p /mnt/EFI/BOOT
cp /boot/loader.efi /mnt/EFI/BOOT/BOOTX64.efi
umount /mnt
This will enable you to boot FreeBSD when booting from the USB drive.
If you want to dual boot, then you need to put both FreeBSD's loader.efi and Windows 10's EFI program on the EFI system partition. Then you could use the program
efibootmgr(8) to add two boot entries to your BIOS and you'll be able to select which OS to boot on startup.
A word of warning - dual booting, especially with Windows, is quite tricky. So you need to be patient to get it right.