For Debian 9 run the following:
Keep in mind that the owner of the key may distribute updates, packages and repositories that your system will trust (more information).
echo 'deb http://download.opensuse.org/repositories/systemsmanagement:/Uyuni:/Stable:/Debian9-Uyuni-Client-Tools/Debian_9/ /' | sudo tee /etc/apt/sources.list.d/systemsmanagement:Uyuni:Stable:Debian9-Uyuni-Client-Tools.list
curl -fsSL https://download.opensuse.org/repositories/systemsmanagement:Uyuni:Stable:Debian9-Uyuni-Client-Tools/Debian_9/Release.key | gpg --dearmor | sudo tee /etc/apt/trusted.gpg.d/systemsmanagement_Uyuni_Stable_Debian9-Uyuni-Client-Tools.gpg > /dev/null
sudo apt update
sudo apt install debhelper