Just had to solve this myself for version 3.0.x so I thought I would just jot this down to save others some time finding it.

PlayKeys.devSettings += "play.server.http.port" -> "8080"

Source: https://www.playframework.com/documentation/3.0.x/ConfigFile#Using-with-the-run-command

Confirmed as working for me on Version 3.0.5 running Java 11.0.15