Issue Composer

[Jürgen_T]

I am running a Drupal-CMS on my Ubuntu-Server managed by PLesk Obsidian, latest version. Using composer what comes with Plesk Obsidian works very fine in the bash i.e. the commands like composer update etc. But using the Plesk U.I. I could not solve the simple problem to tell composer to look in the right directory where my composer.json file resides. Trying to change the location brings up a fixed list of directories but without the right one. As I can not freely select a directory I am now wondering how to tell composer in the Plesk U.I. to look in th right directory. It is not a big problem as I can manage everything from the bash but disturbing.
Any idea how to solve this?