Learn About Router

This will go on a 2.9.x mikrotik where you want to forward a port (tcp 5900) to an internal IP. 69.69.69.69 is the example wan IP, 192.168.1.101 is the desired internal destination.

/ip firewall nat add chain=dstnat dst-address=69.69.69.69 protocol=tcp dst-port=5900 \
   action=dst-nat to-addresses=192.168.1.101 to-ports=5900