How to enable HUD in KDE Plasma to search menu items?

You will need to have either global menu widget enabled, or have app menu button in the titlebar.

• To install the dependencies, run

sudo apt install rofi python3 python3-dbus python3-setproctitle python3-xlib gir1.2-gtk-3.0
sudo apt install appmenu-gtk2-module appmenu-gtk3-module

• Download the source code:

git clone https://github.com/Zren/plasma-hud
cd plasma-hud

• Use the following commands to install the files

sudo mkdir -p /usr/lib/plasma-hud
sudo cp usr/lib/plasma-hud/plasma-hud /usr/lib/plasma-hud/
sudo mkdir -p /etc/xdg/autostart
sudo cp etc/xdg/autostart/plasma-hud.desktop /etc/xdg/autostart/

• Log out and log in.

• Run the following two commands one by one to bind the Alt keys to HUD.

kwriteconfig5 --file ~/.config/kwinrc --group ModifierOnlyShortcuts --key Alt "com.github.zren.PlasmaHUD,/PlasmaHUD,com.github.zren.PlasmaHUD,toggleHUD"
qdbus org.kde.KWin /KWin reconfigure

This answer was based on the README of this project.