Add Domain Normalization Code to settings.php (Drupal)

See Redirecting Multiple Domains to the same Pantheon Drupal site to normalize your urls. For SEO reasons, you only want your site to be available at one URL. You want the url to redirect to your standard URL.

Also, you'll want to redirect HTTP requests to HTTPS. See Pantheon's documentation:  Redirect for SEO and Security

Do not push your domain normalization code to your live environment until your primary domain DNS process is complete and the domain is pointing to Pantheon.