This guy has written about it:

https://levicki.net/articles/2022/12/25/Ghostlight_support_in_Daz_Studio_421126_and_above.php

He has also written a script (it's in the Downloads section) to turn a simple geometry node (i.e. a 2D plane, etc) or an existing render emitter (i.e. built-in light source) into a ghostlight with one click.

Bear in mind that the ghostlight functionality is experimental (read: buggy) at the moment so any weirdness should not be blamed on the script which works just fine but on the Daz Studio and Iray.