Web Messaging SDK proactive message for existing session

Trying to send a proactive notification via the Web Messaging SDK to a guest who has an existing session ID. I’m using webMessagingClient.sendProactiveMessage with the sessionId parameter, but the call returns a 404 Not Found error immediately.

The session is active in Genesys Cloud, and the ID matches what I see in the logs. Is there a specific header I’m missing or is this endpoint only for new sessions?