Influxdb2 http alert documentation

I am trying to create alerts that go to either zammad, gitlab, or matrix via the http alert endpoint. I have issues tying to figure out how messages are sent. there is no place to put json data or headers directly and I am not sure how the message field is sent via http. I tried finding documentation but I all I found was this.
https://v2.docs.influxdata.com/v2.0/monitor-alert/notification-endpoints/create/