Looking at the code here and the screenshot under https://www.omgubuntu.co.uk/2021/01/kde-plasma-5-21-features , it looks like, just like its predecessor nx-firewall (formerly nomad-firewall), plasma-firewall has no UI to pick one of the predefined application profiles (firewalld calls them "services") known to firewalld or UFW, and open the ports for it. Instead, you have to know the port number(s) to create a rule. This makes the KCM impractical to use.

Please support the predefined application profiles of the underlying firewall backend, which make it very easy to open up the ports for a given application.

See also: https://github.com/nx-desktop/nx-firewall/issues/15