How can I have default field values for the node add form as arguments in the URL?
I figured it out.
in .module, hook_form_after()
$value = \Drupal::request()-&gt;query-&gt;get('q');
$value = Html::escape($value);
$form['student']['widget']['#default_value'] = [0 =&gt; $value];

with url /node/add/trip?q=6, where 6 is id of student Oleg