(Riot has been invested in my Automattic and has rebranded to Element. I have changed most instances of Riot to Element to reflect this. The repositories are still at packages.riot.im though.)
Element is an open, private and secure messenger application built with Matrix that allows you to chat, exchange files, make voice or video calls and conference. It also allows you to bridge other networks as well so you can use the Matrix ecosystems to talk in an IRC room as well.
To install Element messenger on KDE neon open Konsole and type,
sudo sh -c "echo 'deb https://packages.riot.im/debian/ bionic main' > /etc/apt/sources.list.d/matrix-riot-im.list" curl -L https://packages.riot.im/debian/repo-key.asc | sudo apt-key add - sudo apt-get update && sudo apt-get -y install riot-web
This will install Element and add it to your sources.list so that it stays updated along with everything else. If you are using an older version of KDE neon (not based on the 18.04 release) then you might have to change the word bionic in the above commands).
Pressing enter after each line is input. You may also have too authenticate with your password and answer some questions. After installation Element can be found in the Application Menu under Applications/Internet or by selecting the menu and typing its name.
If you want to set up Element to use the KDE server the instructions can be found here.