Feature request from mage2pro, posted on GitHub Nov 02, 2015
An event on a backend CMS page form creation:
https://github.com/magento/magento2/blob/f578e54e093c31378ca981cfe336f7e651194585/app/code/Magento/C...
No any event on a backend CMS block form creation: https://github.com/magento/magento2/blob/c743dec47b2e5de036eb5638fec44a54bfb31040/app/code/Magento/C...