For Debian 10 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/home:/Freeman310/Debian_10/ /' | sudo tee /etc/apt/sources.list.d/home:Freeman310.list
curl -fsSL https://download.opensuse.org/repositories/home:Freeman310/Debian_10/Release.key | gpg --dearmor | sudo tee /etc/apt/trusted.gpg.d/home_Freeman310.gpg > /dev/null
sudo apt update
sudo apt install mono-complete