Splunk Management Port (8089) by default use splunk default certificate and when you try to access REST API on port 8089, you'll get certificate warning issue.
To get rid of warning you need to configure internal SSL certificate or 3rd party SSL certificate for port 8089 in server.conf
For the people looking for solution, follow above link and request certificate from CA and update server.conf as below
sslPassword = ********
sslRootCAPath = file path of the CA public certificate
enableSplunkdSSL = true
serverCert = /opt/splunk/etc/auth/New Pem certificate