Initiating cobrowse session via Conversations API returns 400

Trying to start a cobrowse session programmatically using the Genesys Cloud Conversations API. The goal is to trigger it from a backend service when an agent requests screen sharing.

POST to /api/v2/conversations/cobrowse with this payload:

{
 "sessionId": "abc-123",
 "userId": "agent-id-456"
}

Getting a 400 Bad Request. The response doesn’t specify the missing field. Is the endpoint correct for initiating the session, or do I need to use a different method?