I have a queue that uses 192.168.0.0/16 as the target address for the LAN users. However, my firewall outside of the Mikrotik has an IP address of 192.168.168.168 (it's the gateway). How can I add something that won't shape the traffic going to this address coming from the LAN users?

Just create a new queue rule for target 192.168.168.168/32 with unlimited bandwidth. Place this new rule above the 192.168.0.0/16 rule.

/ queue simple
add name="gateway" target-addresses=192.168.168.168/32 dst-address=0.0.0.0/0 \
interface=all parent=none direction=both priority=8 \
queue=ethernet-default/ethernet-default limit-at=0/0 max-limit=0/0 \
total-queue=default-small disabled=no

Ah, makes sense... although I'm not sure how I put this above another rule?

Nevermind I figured it out, thanks.

drag and drop it where ever you want