In Winbox
1) Go to IP -> Firewall -> NAT (Figure 1-1).
Figure 1-1
2) Click the “+” to add a new NAT rule. Modify the “Chain” to “dstnat”, “Protocol” to “tcp”, and “Dst. Port” to “3999”. Set the “In. Interface” to your WAN port. (Note: You are telling the router that any traffic coming IN from the Internet on port 3999 should follow this rule. If you forget this step, the router will grab ANY traffic on port 3999 and send it to the IP you specify in the next step) (Figure 1-2).
Figure 1-2
3) Click the “Action” tab, change the “Action” value to “dst-nat”, the “To Addresses” to “192.168.1.200” and “To Ports” to “3999” (Figure 1-3).
Figure 1-3
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