How to activate application launcher using meta key

Though it might be late but I just faced with the same issue right after upgrading from 18.04 into 19.04. The solution is to right click on your application launcher and choose Configure Application Launcher.... Then in Keyboard shortcuts assign Alt+F1 for it. You might get an alert for conflict with Plasma app shortcut for the same functionality, but just accept it and apply the change.

Now you should be able to use Meta/Super key to open Application Launcher.

Turned out I had modified the file ~/.config/kwinrc when I installed Latte dock and after I removed it, the meta key was no longer activating Plasma's application launcher.

To enable it again open ~/.config/kwinrc and update the following section:

[ModifierOnlyShortcuts]
Meta=org.kde.plasmashell,/PlasmaShell,org.kde.PlasmaShell,activateLauncherMenu