I ran into this recently and hope to help those looking to define a custom HTTP port on Axis products running firmware 7.20+.  I'm not sure if this applies across the board and/or if it was implemented in a version earlier than 7.20  I experienced this on a couple of different models of camera so I'm assuming it's consistent across the board.

This was not easy to find after looking everywhere in the new GUI.  I also was not able to find reference to this being only available in the 'plain config' section anywhere in Axis documentation.  I did some digging and wanted to share where I finally located the setting:

- Expand Settings

- Click System

- Click Plain Config

- Select "System" from drop down

- Locate HTTP Port in the upper area

Screenshots:

Axis 7.20 Firmware Port Forwarding