How to request CMS Hosting Platform service
From the CMS Hosting Platform (CHP) service overview, click the Request button inside the "Request Developer Account" section, and fill out the form to request a Pantheon developer account.
Once you have a developer account, you will be able to create sandbox sites using one of the available distributions. The CHP team recommend taking advantage of one of the custom UT distributions, UT Drupal Kit or WordPress for Texas. You can also select from one of the Pantheon maintained core distributions for Wordpress and Drupal.