How do I figure out the variables for process integration?

I'm trying to send the incident title along with custom fields as a json to power automate.

Everything is sent fine but the fields are not populating with the data from the incident.

I'm trying to send the below, but {{incident_id}} is not a thing appraently, and I have no idea how to find out what actually is? I've tried a multitude of variables inside the brackets {{}}} and the values are never anything to do with the incident.

{
"id": "{{incident_id}}",
"name": "{{incident_name}}",
"custom_fields_values": [
{
"custom_field_id": 4500813,
"user": {
"name": "{{user_name}}",
"email": "{{user_email}}"
}
},
{
"custom_field_id": 4500815,
"value": "{{Date Leaving}}"
}
]
}