Form plugin : how to dynamically initialize form value depending on header data from a plugin

public function onFormInitialized(Event $event)
{
  $form = $event['form'];

  if( $form->getName() != 'my-form' ) {
    return;
  }

  $page = $this->grav['page'];
  $my_data = $page->header()->my_data;
  if( $my_data ) {
    $form->setData('my_data', $my_data);
  }
}

---
title: Contact
variables:
  name: My default value
form:
    name: contact

    fields:
        name:
          label: Name
          placeholder: Enter your name
          autocomplete: on
          type: text
          validate:
            required: true
     ... etc.

public function onPageInitialized(Event $event)
{
    $this->page = $event['page'];
}

public function onFormInitialized(Event $event)
{
    $form = $event['form'];
    if (isset($this->page) && $form->name === 'contact') {
        $form->setData('name.default', $this->page->header()->variables['name']);
    }
}

form:
    name: contact

    fields:
        name:
          label: Name
          placeholder: Enter your name
          autocomplete: on
          data-default@: ['\Grav\Plugin\MyPluginClass::getDefault', 'name']
          type: text
          validate:
            required: true

public function onPageInitialized($event) {
    self::$page = $event['page'];
}

public static function getDefault(string $field) {
    if (isset(self::$page)) {
        return self::$page->header()->variables[$field] ?? '';
    }
}

public function onTwigInitialized(Event $e)
{
  $this->grav['twig']->twig()->addFunction(
    new \Twig_SimpleFunction('set_form_default_values', [$this, 'setFormDefaultValues'])
  );
}

public function setFormDefaultValues($form)
{
  $form->setData(...);
}

{{ set_form_default_values(form) }}

public static function getDefault(string $field) {
    $grav = Grav::instance();

    $pages = $grav['pages'];
    $page = $pages->find('route/of/page');

    return $page ? $page->header()->variables['name'] : '';
}