Simply, by passing it as parameter into the url :

https://mydomain.tld/filemanager/index.html?exclusiveFolder=my/sub/folder/

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

To be clear, if fileRoot = "/home/user/files/", the filemanager will ONLY open the folder /home/user/files/my/sub/folder/.

See also : Passing parameters to the FM

Notice that this method is not safe to prevent users for browsing the default root folder. This is not a permission mechanism. It just applies on displaying files.

Please, do use the connector technique to prevent users to access unwanted/forbidden folders.

The use of exclusiveFolder parameter can be combined with expandedFolder option. Read about expandedFolder option