diff -u b/src/Tests/ComplexWidgetWebTest.php b/src/Tests/ComplexWidgetWebTest.php
--- b/src/Tests/ComplexWidgetWebTest.php
+++ b/src/Tests/ComplexWidgetWebTest.php
@@ -434,7 +434,7 @@
    */
   public function testReferencingExistingEntitiesNoSubmit() {
     // Allow addition of existing nodes.
-    $this->setAllowExisting(TRUE);
+    $this->updateSetting('allow_existing', TRUE);
     $title = $this->randomMachineName();
 
     $this->drupalCreateNode([
only in patch2:
unchanged:
--- a/README
+++ b/README
@@ -32,9 +32,9 @@ Integrating with Inline Entity Form
 An entity type can add support for this module by declaring the
 inline entity form controller class in its entity info:
 
-$entity_info['commerce_line_item']['inline_form'] = array(
+$entity_info['commerce_line_item']['inline_form'] = [
   'controller' => 'CommerceLineItemInlineEntityFormController',
-);
+];
 
 The controller needs to extend EntityInlineEntityFormController and at least
 override entityForm() to provide a functioning entity form.
only in patch2:
unchanged:
--- a/src/Plugin/Field/FieldWidget/InlineEntityFormComplex.php
+++ b/src/Plugin/Field/FieldWidget/InlineEntityFormComplex.php
@@ -3,7 +3,10 @@
 namespace Drupal\inline_entity_form\Plugin\Field\FieldWidget;
 
 use Drupal\Component\Utility\NestedArray;
+use Drupal\Component\Utility\Tags;
+use Drupal\Core\Entity\Element\EntityAutocomplete;
 use Drupal\Core\Entity\EntityDisplayRepositoryInterface;
+use Drupal\Core\Entity\EntityReferenceSelection\SelectionPluginManagerInterface;
 use Drupal\Core\Entity\EntityTypeBundleInfoInterface;
 use Drupal\Core\Entity\EntityTypeManagerInterface;
 use Drupal\Core\Extension\ModuleHandlerInterface;
@@ -36,6 +39,13 @@ class InlineEntityFormComplex extends InlineEntityFormBase implements ContainerF
    */
   protected $moduleHandler;
 
+  /**
+   * Selection Plugin Manager service.
+   *
+   * @var \Drupal\Core\Entity\EntityReferenceSelection\SelectionPluginManagerInterface
+   */
+  protected $selectionManager;
+
   /**
    * Constructs a InlineEntityFormComplex object.
    *
@@ -57,10 +67,13 @@ class InlineEntityFormComplex extends InlineEntityFormBase implements ContainerF
    *   The entity display repository.
    * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
    *   Module handler service.
+   * @param \Drupal\Core\Entity\EntityReferenceSelection\SelectionPluginManagerInterface $selection_manager
+   *   The selection plugin manager.
    */
-  public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, array $third_party_settings, EntityTypeBundleInfoInterface $entity_type_bundle_info, EntityTypeManagerInterface $entity_type_manager, EntityDisplayRepositoryInterface $entity_display_repository, ModuleHandlerInterface $module_handler) {
+  public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, array $third_party_settings, EntityTypeBundleInfoInterface $entity_type_bundle_info, EntityTypeManagerInterface $entity_type_manager, EntityDisplayRepositoryInterface $entity_display_repository, ModuleHandlerInterface $module_handler, SelectionPluginManagerInterface $selection_manager) {
     parent::__construct($plugin_id, $plugin_definition, $field_definition, $settings, $third_party_settings, $entity_type_bundle_info, $entity_type_manager, $entity_display_repository);
     $this->moduleHandler = $module_handler;
+    $this->selectionManager = $selection_manager;
   }
 
   /**
@@ -76,7 +89,8 @@ class InlineEntityFormComplex extends InlineEntityFormBase implements ContainerF
       $container->get('entity_type.bundle.info'),
       $container->get('entity_type.manager'),
       $container->get('entity_display.repository'),
-      $container->get('module_handler')
+      $container->get('module_handler'),
+      $container->get('plugin.manager.entity_reference_selection')
     );
   }
 
@@ -598,12 +612,68 @@ class InlineEntityFormComplex extends InlineEntityFormBase implements ContainerF
     // If the inline entity form is still open, then its entity hasn't
     // been transferred to the IEF form state yet.
     if (empty($values) && !empty($widget_state['form'])) {
-      // @todo Do the same for reference forms.
       if ($widget_state['form'] == 'add') {
         $element = NestedArray::getValue($form, [$field_name, 'widget', 'form']);
         $entity = $element['inline_entity_form']['#entity'];
         $values[] = ['entity' => $entity];
       }
+      elseif ($widget_state['form'] == 'ief_add_existing') {
+        $element = NestedArray::getValue($form, [$field_name, 'widget', 'form'])['entity_id'];
+        if (!empty($element['#value'])) {
+          $options = [
+            'target_type' => $element['#target_type'],
+            'handler' => $element['#selection_handler'],
+            'handler_settings' => $element['#selection_settings'],
+          ];
+          /** @var \Drupal\Core\Entity\EntityReferenceSelection\SelectionInterface $handler */
+          $handler = $this->selectionManager->getInstance($options);
+          $input_values = $element['#tags'] ? Tags::explode($element['#value']) : [$element['#value']];
+
+          foreach ($input_values as $input) {
+            $match = EntityAutocomplete::extractEntityIdFromAutocompleteInput($input);
+            if ($match === NULL) {
+              // Try to get a match from the input string when the user didn't use
+              // the autocomplete but filled in a value manually.
+              $entities_by_bundle = $handler->getReferenceableEntities($input, '=');
+              $entities = array_reduce($entities_by_bundle, function ($flattened, $bundle_entities) {
+                return $flattened + $bundle_entities;
+              }, []);
+              $params = [
+                '%value' => $input,
+                '@value' => $input,
+              ];
+              if (empty($entities)) {
+                $form_state->setError($element, t('There are no entities matching "%value".', $params));
+              }
+              elseif (count($entities) > 5) {
+                $params['@id'] = key($entities);
+                // Error if there are more than 5 matching entities.
+                $form_state->setError($element, t('Many entities are called %value. Specify the one you want by appending the id in parentheses, like "@value (@id)".', $params));
+              }
+              elseif (count($entities) > 1) {
+                // More helpful error if there are only a few matching entities.
+                $multiples = [];
+                foreach ($entities as $id => $name) {
+                  $multiples[] = $name . ' (' . $id . ')';
+                }
+                $params['@id'] = $id;
+                $form_state->setError($element, t('Multiple entities match this reference; "%multiple". Specify the one you want by appending the id in parentheses, like "@value (@id)".', ['%multiple' => implode('", "', $multiples)] + $params));
+              }
+              else {
+                // Take the one and only matching entity.
+                $values += [
+                  'target_id' => key($entities),
+                ];
+              }
+            }
+            else {
+              $values += [
+                'target_id' => $match,
+              ];
+            }
+          }
+        }
+      }
     }
     // Sort values by weight.
     uasort($values, '\Drupal\Component\Utility\SortArray::sortByWeightElement');