I have tried this code but it is not working.

```
$form['field_test'] = [
  '#title' => t('test'),
  '#type' => 'radios',
  '#options' => [t('A'), t('B'), t('C')],
 ];
$form['#group_children']['field_test'] = 'group_1';

```

I also tried with 

```
$form['#fieldgroups']['group_1']->children[] = 'field_test';

```

But nothing is working. It works in Drupal 7. Is it changed in Drupal 8?

I have written this code in `hook_form_alter` and I'm altering a node form.