Web Messaging SDK: Sending proactive notifications to returning customers?

Hey folks,

We’re trying to send a proactive notification to a user who already has a recent web messaging session. The SDK docs mention createProactiveNotification, but I’m not sure how to link it to the existing guest context without starting a brand new conversation. Here’s what I’ve got so far:

const notification = {
 message: 'Hey, still need help?',
 guestId: 'existing-guest-id-here' // This feels wrong
};
client.conversations.proactive.createProactiveNotification(notification);

It just errors out with a 400. Is there a specific API endpoint to query the guest’s active session ID first?