Complete the follow steps to obtain the Whonix Signing Key. Download it to the home folder using Tor Browser, or alternatively download it through Whonix and recover it from the shared folder.

Add Whonix's Signing Key to apt-key.

gpg --export 916B8D99C38EAF5E8ADC7A2A8D66066A2EEACCDA | sudo apt-key add -

Install apt-transport-tor from the Debian repository.

sudo apt-get install apt-transport-tor

Add Whonix's APT repository for default Whonix using Debian stable. At the time of writing this was Jessie.

echo "deb tor:// jessie main" | sudo tee /etc/apt/sources.list.d/whonix.list

For greater security, set apt to use the Whonix Onion repository instead.

echo "tor://deb.kkkkkkkkkk63ava6.onion/ jessie main" | sudo tee /etc/apt/sources.list.d/whonix.list