Ubuntu

(tested with Gnome > Ubuntu 20.04)

1. Install sddm from terminal by running the following command sudo apt istall sddm

2. right after the installation, you should be prompted to a dialogue box saying you have to set your default Display Manager, select sddm instead of the others you already have (e.g. Gnome DE has gdm3 as default) (if the dialogue box doesn't automatically shows or you missed it look here)

3. look for a sddm.conf file in /etc and edit it so that it looks as shown below (if it doesn't exist, look here):

[Theme]
# Current theme name
Current=sddm-theme-amadeus-master 

4. edit the Main.qml file in this folder by opening it (if it doesn't let you open it just rename .qml with .txt, edit with any text editor, then rename the .qml back again ;D) and delete the following lines

import QtQuick.VirtualKeyboard 2.1

InputPanel {
    id: inputPanel
    active: false
    visible: false
}

5. Install the Qt modules you're lacking off (otherwise you'll get an error on greeter screen) from the terminal by running the following command

apt install qml-module-qtquick-virtualkeyboard qml-module-qtgraphicaleffects