Create new zone and add sources and services:

firewall-cmd --permanent --new-zone=monitoring
firewall-cmd --permanent --zone monitoring --add-source=<monitoring source address>
firewall-cmd --permanent --zone monitoring --add-service=<your services>
firewall-cmd --reload

Get zones:

firewall-cmd --get-zones

Set default zone:

firewall-cmd --set-default-zone=internal

Verify:

firewall-cmd --get-zone-of-interface=eth0

Stop firewalld:

systemctl stop firewalld.service

Create service file:vi /etc/firewalld/services/splunk.xml

<?xml version="1.0" encoding="utf-8"?>
<service version="1.0">
  <short>splunk</short>
  <port port="8000" protocol="tcp"/>
</service>

Start firewalld:

systemctl start firewalld.service

Add service to zone:

firewall-cmd --permanent --zone=internal --add-service=splunk

Restart firewalld:

systemctl restart firewalld