Horizon Core has an embedded ActiveMQ instance for convenience that you can enable. It uses the same credentials configured from the web UI for users in the ROLE_MINION role.

Edit ActiveMQ configuration file
sudo vi etc/opennms-activemq.xml
Remove comments for the transport connector listening on 0.0.0.0 and save
<transportConnector name="openwire" uri="tcp://0.0.0.0:61616?useJmx=false&amp;maximumConnections=1000&amp;wireformat.maxFrameSize=104857600"/>
Restart Horizon Core instance
sudo systemctl restart opennms
Verify that ActiveMQ port is available on public network interface
ss -lnpt sport = :61616
Verify listening 61616/tcp on all interfaces
State   Recv-Q  Send-Q  Local Address:Port  Peer  Address:Port
LISTEN  0       128     *:61616             *:*   users:(("java",pid=1,fd=706))
If you run a host firewall, allow port 61616/tcp, as in the following example with firewalld:
sudo firewall-cmd --permanent --add-port=61616/tcp
sudo firewall-cmd --reload