--- /dev/null
+/**
+ * Implements hook_field_widget_multivalue_WIDGET_TYPE_form_alter().
+ */
+function {{ machine_name }}_field_widget_multivalue_WIDGET_TYPE_form_alter(array &$elements, \Drupal\Core\Form\FormStateInterface $form_state, array $context) {
+ // Code here will only act on widgets of type WIDGET_TYPE. For example,
+ // hook_field_widget_multivalue_mymodule_autocomplete_form_alter() will only
+ // act on widgets of type 'mymodule_autocomplete'.
+ // Change the autocomplete route for each autocomplete element within the
+ // multivalue widget.
+ foreach (Element::children($elements) as $delta => $element) {
+ $elements[$delta]['#autocomplete_route_name'] = 'mymodule.autocomplete_route';
+ }
+}