From bare domain to www for D8

settings.php

<?php

########
if (isset($_ENV['PANTHEON_ENVIRONMENT']) && php_sapi_name() != 'cli') {
    // Redirect to https://$primary_domain in the Live environment
    if ($_ENV['PANTHEON_ENVIRONMENT'] === 'live' && ($_SERVER['HTTP_HOST'] !== 'live-psea.pantheonsite.io')) {

        if ($is_www = strpos($_SERVER['HTTP_HOST'], 'www')) {
            $primary_domain = $_SERVER['HTTP_HOST'];
        } else
            $primary_domain = 'www' . $_SERVER['HTTP_HOST'];
        /** Replace www.example.com with your registered domain name */
    } else {
        // Redirect to HTTPS on every Pantheon environment.
        $primary_domain = $_SERVER['HTTP_HOST'];
    }

    if ($_SERVER['HTTP_HOST'] != $primary_domain || !isset($_SERVER['HTTP_USER_AGENT_HTTPS']) || $_SERVER['HTTP_USER_AGENT_HTTPS'] != 'ON') {

        # Name transaction "redirect" in New Relic for improved reporting (optional)
        if (extension_loaded('newrelic')) {
            newrelic_name_transaction("redirect");
        }

        header('HTTP/1.0 301 Moved Permanently');
        header('Location: https://' . $primary_domain . $_SERVER['REQUEST_URI']);
        exit();
    }
    // Drupal 8 Trusted Host Settings
    if (is_array($settings)) {
        $settings['trusted_host_patterns'] = array('^' . preg_quote($primary_domain) . '$');
    }
}