1.
You first have to import my public GPG key because all of my pools are signed. Run this once:
wget -q -O - https://mkvtoolnix.download/gpg-pub-moritzbunkus.txt | sudo apt-key add -

2.You can access the packages by adding the appropriate lines below to a file such as 
/etc/apt/sources.list.d/mkvtoolnix.download.list:

18.04 "Bionic Beaver"

deb https://mkvtoolnix.download/ubuntu/ bionic main
deb-src https://mkvtoolnix.download/ubuntu/ bionic main


3.
Now simply run 
sudo apt update 
followed by 
sudo apt install mkvtoolnix mkvtoolnix-gui.

If updating the package lists fails, make sure you have the package apt-transport-https installed.