Populate coupon code on signup page from PHP session

First, ensure that the signup form includes the Coupon brick. Then add the following code to the site.php file:

$_SESSION['signup_coupon'] = 'TEST';

Am_Di::getInstance()->hook->add(Am_Event::BEFORE_RENDER, function (Am_Event $event) {
    $template = str_replace('\\', '/', $event->getTemplateName());
    if (strpos($template, 'signup/signup') === false) {
        return;
    }

    $couponCode = $_SESSION['signup_coupon'] ?? '';
    $view = $event->getView();
    if (!$couponCode || empty($view->form)) {
        return;
    }

    $elements = $view->form->getElementsByName('coupon');
    if ($elements && !$elements[0]->getValue()) {
        $elements[0]->setValue($couponCode);
    }
});

Replace TEST with the coupon code you want to populate. If another script already stores the coupon code in the PHP session, remove the first line and use that script's session key instead of signup_coupon.