HOWTO : Get A+ Rating on SSLLabs - Using Sentora
We presume you have already followed this one
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...