1) Add the code of config.inc.php.dist to config.inc.php or config.inc.php.dist of markasjunk plugin
   with your settings, following the instructions to create ispconfig remote user.
2) Set "$config['markasjunk_learning_driver'] = 'ispconfig_spam';" in config.inc.php of markasjunk.
3) Copy the driver to the drivers dir in markasjunk plugin.
4) Add "markasjunk" in plugins list of roundcube main config.inc.php