/**
 * 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';
  }
}