TLDR
====

sudo apt install -y apt-transport-https lsb-release
sudo mkdir -p /etc/apt/keyrings
sudo wget -qO /etc/apt/keyrings/devys.asc https://deb.devys.org/devys.asc
sudo tee /etc/apt/sources.list.d/devys.sources <<EOF
Types: deb
URIs: http://deb.devys.org/debian
Suites: $(lsb_release -cs)
Components: main
Signed-By: /etc/apt/keyrings/devys.asc
EOF

Longer explanations
===================

You need to install https support for apt as the repository is not available
in http::

    sudo apt install apt-transport-https

Then install the public key to verify packages::

    sudo mkdir -p /etc/apt/keyrings
    sudo wget -qO /etc/apt/keyrings/devys.asc https://deb.devys.org/devys.asc

Then add this in /etc/apt/sources.list.d/devys.sources::

    Types: deb
    URIs: http://deb.devys.org
    Suites: stable (or your codename suite, you can get it with lsb_release -cs)
    Components: main
    Signed-By: /etc/apt/keyrings/devys.asc