Dmix, asound.conf, route playback to headphones when multiple cards installed

I am having a challenge with dmix on RPi4 Raspbian Buster. I can select my HiFiBerry add-on board and Audacity 2.2.2 records and plays recorded audio over that device, but when I want to route playback to dmix specifically to the on-board headphones, it does not play. I would like to specify the specific interface card and device.

If I was to create an asound.conf file for ALSA, using dmix and a slave, are you looking for a specific name like ‘default’?

pcm.!default {
type plug
slave.pcm “dmixer”

I thought I would be able to get away with not have an asound.conf file, and toggle using the Audio Output selection on the panel.