I would like to modify the form class so for every field_reference it adds a button + so users are able to directly insert a new element into that referenced table.
i.e: I'm completing a customer form and cannot found his city in the list. I just click an the + and the city form appears and let me insert the correct city.
I think I have to try by modifying atk4/lib/Controller/MVCForm.php around
if($field instanceof Field_Reference || $field_type=='reference') {
$form_field->setModel($field->getModel());
}
But maybe it's something to add in atk4/lib/field/Reference.php directly.
Try out this one: https://github.com/atk4/autocomplete/blob/master/lib/Form/Field/Plus.php
It's not exactly what you're asking for, but this field type has implemented "plus" behavior. You can at least get some ideas from there if not using autocomplete field