User patovm04 from the FreeBSD forums shows us how to dual-boot FreeBSD with ZFS along with another operating system. Check out their guide on the FreeBSD forums below.

Important notes:
1) This tutorial assumes you have the OS you want to dual-boot with already installed on your drive, and that you already have freed up some disk space. Essentially, you will be installing FreeBSD with root-on-ZFS on the remaining free space of the disk, instead of using the entire drive as it’s done by default by FreeBSD’s installer.

Well, let’s start already:
First of all, boot your FreeBSD install USB stick. Proceed installing as usual, until you reach the “Partitioning” stage. Here, choose the “Shell” option, so you can create your swap and ZFS partitions by hand:

# Load the ZFS kernel module:
kldload zfs

[UEFI/GPT] [Dual-Boot] How to install FreeBSD (with ZFS) alongside another OS (sharing the same disk):