The simplest way is to install pavucontrol, open the application and change the audio output selection for the application. It should default to that afterwards.
You can look at ~/.local/state/wireplumber/restore_streams to see the changes.
As an example, I’ve got mpv set to play out of my headphones currently. Output/Audio:application.name:mpv:target=alsa_output.usb-Logitech_Logitech_G430_Gaming_Headset-00.analog-stereo
I’m guessing there is another way to do this but I haven’t found it yet.