POST /api/v2/conversations/cobrowse returns 400 with 'session not found'

NICE CXone API & Integrations
1

Trying to kick off a cobrowse session via the API from a custom desktop app.

POST /api/v2/conversations/cobrowse
{
 "participantId": "user-123",
 "applicationId": "app-456"
}

Getting a 400 Bad Request saying the session doesn’t exist. I’ve checked the participant is active. Is there a pre-flight step I’m missing to create the session object before triggering the conversation?