sports_league-8.x-1.x-dev/modules/sl_transfer/sl_transfer.module

<?php

/**
 * @file
 * The SL standinds module.
 */

/**
 * Implements hook_inline_entity_form_entity_form_alter().
 */
function sl_transfer_inline_entity_form_entity_form_alter(&$entity_form, &$form_state) {

  if ($entity_form['#entity_type'] == 'sl_transfers') {
    $parent_entity = $form_state->getFormObject()->getEntity();
    $entity_form['field_sl_transfer_player']['widget'][0]['target_id']['#default_value'] = $parent_entity->id();
    $entity_form['field_sl_transfer_player']['widget'][0]['target_id']['#type'] = 'hidden';
  }

}