Web Messaging widget: passing custom guest attributes via init config

Trying to pass a userId to the Genesys Cloud web widget for authenticated users. The genesys.cloud.messenger.init call doesn’t seem to accept a guestAttributes object in the config, and the setAttributes method isn’t exposed on the returned instance. Here’s the snippet:

genesys.cloud.messenger.init({
 deploymentId: 'abc123',
 region: 'us-east-1',
 guestAttributes: {
 userId: 'user-99'
 }
});

The widget loads but the attribute never shows up in the conversation context. How do we inject this?