Installation of Virtualbox 6 on Linux Mint 19 (based on Ubuntu Betsy):

wget -q https://www.virtualbox.org/download/oracle_vbox_2016.asc -O- | sudo apt-key add -
echo "deb [arch=amd64] http://download.virtualbox.org/virtualbox/debian bionic contrib" | sudo tee /etc/apt/sources.list.d/virtualbox.list
sudo apt-get update
sudo apt-get install virtualbox-6.0

For Debian 9 / LMDE 3 replace bionic with stretch in the apt line above.