How do I set up port forwarding?

I'm writing a server for a game I want users to connect to. Because I'm using the Godot engine, I cannot listen to a port directly but instead have to have port forwarding set up so I can listen for local connections. When I run my server on my Windows machine I can do this by configuring the router to forward external connections on a certain port to my local network. But now I'm trying to get this to work on a VPS running Ubuntu.

I'm not a terribly advanced Linux user. What can I do to forward external connections on port 13650 to my local network?