$modal_form = Drupal::formBuilder()->getForm('Drupalmy_moduleFormModalForm');

$element['#value']['fids'][] = $fid;
$element['#value']['target_id'] = $fid;
$element['#display_default'] = TRUE;
$element['fids']['#value'] = [
  $fid,
];

// At the end of my formElement() function on my field widget form
$session = new Session(); // Symfony session
$session->set('my_module.image_widget_form_state', $form_state); // save the $form_state object

// Later...
$image_widget_form_state = $session->get('my_module.image_widget_form_state');
$image_widget_form_state->setValue('fids', [$fid]); // attempt to update $form_state at a later stage from within a different form