Yes, thank you, it works!
I have created a service custom attribute “OPOMBA” and Custom host attribute “CHA_NOTIFICATION_HELPER1” which I included in ~/etc/nagios/conf.d/check_mk_templates.cfg like this:
NOTIFY_HOST_ADDRESS_FAMILY='$_HOSTADDRESS_FAMILY$' \
NOTIFY_SERVICE_CSA_OPOMBA='$_SERVICEOPOMBA$' \
NOTIFY_HOST_CHA_NOTIFICATION_HELPER='$_HOSTCHA_NOTIFICATION_HELPER1$' \
check_mk --notify
}
Important thing to mention is that, within the context of notification, the names of these variables are SERVICE_CSA_OPOMBA and HOST_CHA_NOTIFICATION_HELPER. So, when I included them in the e-mail subject, I used the following sytax:
$SERVICE_CSA_OPOMBA$ for SERVICE_CSA_OPOMBA
$HOST_CHA_NOTIFICATION_HELPER$ for HOST_CHA_NOTIFICATION_HELPER
So, a lot of tiny details that you need to configure correctly. But in the end it works.