Not able to send dynamic content(log messages) in Kibana Alerts ( v 8.13.2)

we are sending alerts to overture from kibana logs. Every matching log should create an alert and we want to see the log message in alert.
below is the alert body, I have tried several things but could not get the error message in the alert:

{
"id": "13213123",
"specversion": "1.0",
"type": "com.xyz",
"source" : "Logs",
"datacontenttype" : "application/json",
"data": {
"severity": "medium",
"startsAt": "{{ctx.timestamp}}",
"environment": "test",
"alertname": "{{rule.name}}",
"message": "Log alerts triggered:{{ctx.payload.hits.hits.0._source.message}}",
"summary": "{{rule.name}}.",
"tags": "{{tags}}"
}
}