How to open a given folder different from root folder when opening the filemanager?

Simply, by passing it as parameter into the url :

http://mydomain.tld/filemanager/index.html?expandedFolder=my/sub/folder/

The given path should take the value from fileRoot folder value.

To be clear, if fileRoot = "/home/user/files/", that means the desired folder, in the example above, is located into /home/user/files/my/sub/folder/.