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 and macOS:
git clone git@github.com:texmacs/maxima.git $HOME/.TeXmacs/plugins/maxima
For Windows:
git clone git@github.com:texmacs/maxima.git %APPDATA%\TeXmacs\plugins/maxima