It is a built-in plugin in TeXmacs. If you need to customize it, please install it
under the $TEXMACS_HOME_PATH:
# For GNU/Linux or macOS
git clone git@github.com:texmacs/octave.git $HOME/.TeXmacs/plugins/octave
# For Windows
git clone git@github.com:texmacs/octave.git %APPDATA%\TeXmacs\plugins/octave