How To Open a Port
You can open a port by heading to the Network & Ports page under Advanced. Here you can open a random port for your server by pressing the Open Port button.
In most cases, you don't need a set number, as the port number can be changed in the configs. Minecraft plugins for example always have a config you can set the port number in. Random ports are used unless you have a dedicated IP, more than one client will be on the same IP.
If you own a dedicated IP, you can set a number of your choosing by opening a support ticket.