How to pass values to content creation page to set default values in form elements?
Altering the form

You should implement the hook_form_FORM_ID_alter. Then, read the parameters like this:

$name = \Drupal::request()-&gt;request-&gt;get('country'); // from param


Then alter the form elements, rewriting the #defatult_value conditionally, like this:

$form['field_country']'#default_value'] = 'my domain specific default';


In the case of a checkbox widget, the #default_value is an array (source):

$form['countries']['#default_value'] = [1, 3];

Passing parameters in general

If your parameters are simple strings, like country: denmark, you could use GET parameters. This answer has the sample code:

How do I create a link?

Passing parameters through the '/node/add' route

This is a special route, it is not so simple.

Method 1: Implementing a subtheme

If you are using the default admin theme ('seven') you would have to create a subtheme, and overwrite this hook, so that your custom parameters are passed:

   /**
     * Implements hook_preprocess_HOOK() for list of available node type templates.
     */
    function seven_preprocess_node_add_list(&amp;$variables) {
      if (!empty($variables['content'])) {
        /** @var \Drupal\node\NodeTypeInterface $type */
        foreach ($variables['content'] as $type) {
          $variables['types'][$type-&gt;id()]['label'] = $type-&gt;label();
          $variables['types'][$type-&gt;id()]['url'] = \Drupal::url('node.add', ['node_type' =&gt; $type-&gt;id()]);
        }
      }
    }


Method 2: Overwriting the twig template

The links are ultimately rendered by the  core/themes/seven/templates/node-add-list.html.twig template. To overwrite this output, you would: 
 - Implement a hook_theme_suggestions_HOOK
 - Copy the node-add-list.html.twig to node-add-list--custom.html.twig and implement your custom changes here.

/**
 * Implements hook_theme_suggestions_HOOK()
 */
function hook_theme_suggestions_node_add_list(array $variables) {
  $suggestions = array();
  $suggestions[] = 'node_add_list__custom';
  return $suggestions;
}