Google chat bot send a message to a specific user in a room

Question

I have a working google chatbot that can send a message to a specific room that the chatbot is subscribed to using the following code:

$client = new Google_Client();
$client->setAuthConfig('../service-account.json');
$client->addScope('https://www.googleapis.com/auth/chat.bot');

$service = new Google_Service_HangoutsChat($client);

$message = new Google_Service_HangoutsChat_Message();

$message->setText('@Leon Vismer Testing a message.');
$message->setAnnotations([$annotation]);

$service->spaces_messages->create('spaces/SPACE_REFERENCE', $message);

However what I am trying todo is direct the message to a specific user inside the room using a mention. I tried using annotations but that does not seem to work. I get a notification of a new message inside the room but the @mention is not a proper annotation to the specific user.

I tried using:

$user = new Google_Service_HangoutsChat_User([
    'name' => 'users/NUMBER_TO_THE_USER',
    'displayName' => 'Leon Vismer',
    'type' => 'HUMAN',
]);

$annotation = new Google_Service_HangoutsChat_Annotation([
    'type' => 'USER_MENTION',
    'startIndex' => 1,
    'length' => strlen('@Leon Vismer'),
    'userMention' => new Google_Service_HangoutsChat_UserMentionMetadata([
        'user' => $user,
        'type' => 'MENTION',
    ])
]);

Can a chatbot in a way initiate a conversation to a specific user like this?