Terminal:
/ip firewall nat add chain=dstnat dst-address=69.69.69.69 protocol=tcp dst-port=3389 \
action=dst-nat to-addresses=192.168.1.101 to-ports=3389
Winbox:
Go to IP>Firewall and click on NAT tab. Then click new (+)

Chain: dstnat
Dst. Address: WAN IP
Protocol: tcp (change to your need)
Dst Port: 3389 (change to the port you need to forward)
Then click on Action tab.

Action: dst-nat
To Address: 192.168.1.101 (change to internat IP you will be pointing port to)
To Ports: 3389 (change to your needs)
Click OK.

Your port is now forwarded to internal IP.