[#3413340]: Support add_existing_widget from [#2683125]

Closes #3413340

src/Plugin/Field/FieldWidget/InlineEntityFormComplexOpen.php

@@ -316,6 +316,8 @@ class InlineEntityFormComplexOpen extends InlineEntityFormComplex {
           '#entity_type' => $target_type,
           '#ief_labels' => $this->getEntityTypeLabels(),
           '#match_operator' => $this->getSetting('match_operator'),
+          '#ief_items' => $items,
+          '#ief_add_existing_widget' => $this->getSetting('add_existing_widget'),
         ];
         $element['form'] += inline_entity_form_reference_form($element['form'], $form_state);