According to announcements you need to add the corresponding Plasma Widget or the menu button from "Window Decorations" settings.
Global Menus have returned. KDE's pioneering feature to separate the menu bar from the application window allows for a new user interface paradigm, with either a Plasma Widget showing the menu, or with the menu neatly tucked away in the window title bar. Setting it up has been greatly simplified in Plasma 5.12: as soon as you add the Global Menu widget or title bar button, the required background service gets started automatically. No need to reload the desktop or click any confirmation buttons!
Step by step instructions:
From the Application Dashboard/Launcher/Menu, select "Settings" (in the Launcher: under "Applications") → "System Settings" → "Application Style" → "Window Decorations" → "Titlebar Buttons"; then drag and drop the "Application menu" icon to the titlebar.