There is no single online platform that covers Montenegro beach reservations the way hotel booking sites do. Most beach bars and clubs do not offer a formal online booking widget — instead, direct contact via WhatsApp, Instagram, or a phone call is the standard method for securing a spot.
A few higher-end beach clubs near Budva and the Sveti Stefan area have begun accepting email or Instagram DM reservations for premium cabanas, but you should expect a manual confirmation process rather than an instant booking. For regular sunbeds on public stretches like Mogren or Becici, online booking simply does not exist — walk up and pay on arrival.