cometd.init(config);
cometd.subscribe('/event/platform_event__e', function(message) {
    // Code to display warning message to user
});

cometd.configure(config);
cometd.addListener('/meta/handshake', function(message) {
    cometd.subscribe('/event/platform_event__e', function(message) {
        // Code to display warning message to user
    );
});
cometd.handshake();