It will look something like this:
- alias: 'Activate siren'
trigger:
platform: state
entity_id: alarm_control_panel.your_alarm
to: 'triggered'
action:
service: mqtt.publish
data:
topic: zigbee2mqtt/your_siren/set_message
payload: '{"warning": {"duration": 10, "mode": "emergency", "strobe": false}}'
You will need to replace your_alarm and your_siren with the names of your alarm panel and Heiman siren.