Some dynamicDNS clients such as DynSite include a "default port" option in which you can enter the port number you wish to forward to. This will only work for webservers, however, as the forwarding for mailservers has to be set up differently

If you are using this method the only other thing you would need to do is login to the members page during the setup and manually enter an ip with the port number following it, like "23.23.23.23:8080."

This lets our system know that it will need to accept updates in that form and pass them through to the forwarding server, and you will only need to do so the first time. After that the dynamic updates will take care of it on their own.