How to map the .Alerts.Firing json object in Grafana Alerting to a Custom structure for PagerDuty Ingestion

I am trying to integrate grafana with PagerDuty to present alerting data structure in a particular way.
I am trying to use the Notification Template to emit a custom json structure for the Alert for ingestion into PagerDuty.
The current object that is forwarded to Pagerduty is a firing object, e.g:

  "details": {  
    "firing": { alert kv's here }

I want to map this in its place:

  "payload": {
    "custom_field": { alert kv's here }

I have been trying to update the notification template to map these objects but struggling to find the right combination. Any help appreciated.