We presume you have already followed this one
http://support.randomsolutions.nl/072480-HOWTO--Sentora-on-Centos-Letsencrypt-with-Python-27
Do this
vi /etc/sentora/configs/apache/httpd-vhosts.conf
Look for the vhost that you want and this line between the <virtualhost things>
<VirtualHost *:443> Header always set Strict-Transport-Security "max-age=63072000; includeSubDomains" </VirtualHost>
you may also just go the vhost in the hostpanel
Admin -> Module Admin -> Apache Config > Override a Virtualhost [Select VHost] ->
Add this line to your Custom Entry:
Header always set Strict-Transport-Security "max-age=63072000; includeSubDomains"