Configuration is done completely though rc.conf.
The rc script won't touch any devices for you (neither disk, nor tap)
so you need to make sure all of those have been initialized properly.

General setup:
kldload vmm
net.link.tap.up_on_open=1

Make it persistent:
echo "net.link.tap.up_on_open=1" >> /etc/sysctl.conf
cat >> /boot/loader.conf << EOF
vmm_load="YES"
EOF

Minimal example:
cat >> /etc/rc.conf << EOF
cloned_interfaces="tap0 bridge0"
bhyve_enable="YES"
bhyve_diskdev="/dev/zvol/tank/bhyve/virt"
EOF

ifconfig tap0 create
ifconfig bridge0 create

service bhyve start
tmux list-sessions
tmux attach -t bhyve
service bhyve status
service bhyve stop

Multi profile configuration example:
cat >> /etc/rc.conf << EOF
cloned_interfaces="tap0 tap1 bridge0"
bhyve_enable="YES"
bhyve_profiles="virt1 virt2"
bhyve_virt1_diskdev="/dev/zvol/tank/bhyve/virt1"

bhyve_virt2_tapdev="tap1"
bhyve_virt2_diskdev="/dev/zvol/tank/bhyve/virt2"
bhyve_virt2_memsize="8192"
bhyve_virt2_ncpu="4"
EOF

ifconfig tap0 create
ifconfig tap1 create
ifconfig bridge0 create

service bhyve start # start all
service bhyve start virt2 # start individual
tmux attach -t bhyve_virt1
tmux attach -t bhyve_virt1
service bhyve stop virt2 # stop individual
service bhyve stop # stop all

(by default ctrl-b d detaches from tmux).