The content type is unfortunately not described in the V2 docs, I assume that the HTML type renders the body?
if (notificationHandler != null) {
// Create Notification for Admins/Supporters.
if(!ticketIO.getPriority().equals(TicketPriority.LOW)) {
List<User> stuffsIO = UserHandler.getStuffs();
List<String> usernamesIO = UserHandler.getUsernames(stuffsIO);
// Send Notification to Online Stuff Members.
notificationHandler.createNotification().withSubject(ticketIO.getName()).withRecipientUsernames(usernamesIO).toChannelsByNames("in-app").withHtmlContentType().withBody("<a href=\"/chat/?id=" + ticketIO.getId() + "\">"+ ticketIO.getId() + "</a>").send();
}
}