privateMessageViewer property
Input for creating a message, otherwise output only.
The user that can view the message. When set, the message is private and only visible to the specified user and the Chat app. Link previews and attachments aren't supported for private messages. Only Chat apps can send private messages. If your Chat app authenticates as a user to send a message, the message can't be private and must omit this field. For details, see Send private messages to Google Chat users.
Immutable.
Implementation
User? privateMessageViewer;