Multiboot Windows10/FreeBSD-11

You don't need grub2 for a windows 10 / FreeBSD 11 dual boot. Install windows 10, shrink the NTFS volume, install FreeBSD with guided partitioning, boot into FreeBSD, run boot0cfg -B /dev/ada0 (or whatever your boot disk device is), then reboot, and you should be presented with a prompt to boot either Windows or FreeBSD.
In either case, you need to get into the habit of explaining more. "It doesn't work" doesn't explain anything.

Are you able to boot?
Does anything display during boot0?
etc, etc.
I've used boot0cfg -B /dev/ada0 but it still boots on Windows10.
It doesn't show any choices either FreeBSD or Windows.
There are some misunderstandings here.

Windows 10 installs are mostly on newer computers with UEFI. If it has UEFI, it has GPT partitioning. boot0 will not work on that at all. It can't, it doesn't know how to deal with a partition format that came along something like twenty years later. The bad thing is that having installed it now, it has overwritten some of the PMBR. It might not matter, since UEFI ignores the MBR bootcode anyway.

If the computer was an upgrade from Windows 7, it could still be using MBR, in which case boot0cfg(8) would work.

For dual-booting with Windows 10, the UEFI firmware might allow setting up boot options. The latest Dells I've bought all have this.
Otherwise, yes, you do need Grub. See

As far as the very first post, "can't install" tells us nothing. Specifically, what did you do and what happened?