Forms: how to filter a related child lookup (Kotlin)


I am trying to figure out how to have a related Sites (entity) be filtered on a form, after the Customer has already been selected on create. Also obviously on Edit to ensure that the filter is maintained so that only related Sites can be selected.

I tried to make sense of this section for my purposes: Initial Entity Values, but besides being Java instead of Kotlin (which I am using) the setting of the value would probably serve as a parameter for a filter on the child control.

Can anyone provide me with a link to documentation, explanation and even better an example in Kotlin?