Proactive message to past guest returns 403 Forbidden

Trying to send a proactive message to a guest from a previous session using the Guest API. The docs say to POST to /api/v2/conversations/messaging/conversations/{conversationId}/guests/{guestId}/messages but I’m getting a 403 Forbidden. Here’s the payload:

{
 "messageType": "text",
 "text": "Welcome back"
}

The conversation is closed. Is there a specific scope I’m missing?