This post is how to setup a static route to Google DNS using 192.168.60.1 as the next hop gateway. You can adapt the dst-address to whatever subnet you need to add routes for.
/ip route
add check-gateway=ping disabled=no distance=1 dst-address=8.8.8.8/32 gateway=\
192.168.60.1 pref-src=0.0.0.0 routing-table=main scope=30 \
suppress-hw-offload=no target-scope=10