Ended up posting a different question and got a response which answer my question:
So ended up needing to get back the transition id using:
{{host}}/rest/process-instance/{{processid}}/activity-instances
look for the the childTransitionInstances id
Then using that you can use:
{{host}}/rest/process-instance/{{processid}}/modification
with payload:
"skipCustomListeners": true,
"skipIoMappings": true,
"instructions": [
{
"type": "cancel",
"transitionInstanceId": "{{transitionid}}"
}
]
}
That essentially cancels the incident