For Debian 12 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:/stevenpusser:/lsp-plugins/Debian_12/ /' | sudo tee /etc/apt/sources.list.d/home:stevenpusser:lsp-plugins.list
curl -fsSL https://download.opensuse.org/repositories/home:stevenpusser:lsp-plugins/Debian_12/Release.key | gpg --dearmor | sudo tee /etc/apt/trusted.gpg.d/home_stevenpusser_lsp-plugins.gpg > /dev/null
sudo apt update
sudo apt install mkvtoolnix-gui