How to prepopulate a node add form?

I am trying to port over a D7 module and am stuck on how to do this step. How can I enter values into a node add form (that the user can override if desired)?

The code sets the [#default_value] for the form field, i.e.

  foreach ($isbn2book_data as $field => $default_value) {
    switch ($field) {
      case 'title':
        $form[$field]['#default_value'] = $default_value;
        break;
      default:
        $form[$field]['und']['0']['value']['#default_value'] = $default_value;
        break;
    }
  }

It works in D7. The form object appears to have the same structure in Backdrop, but the (converted) code doesn't work on it. If I print a debug message with the value of the $form object, I can see that the default values are set correctly, but they don't show up on the form display.

Any help appreciated!

(Following up on the book data module topic here: https://forum.backdropcms.org/forum/book-data-module)