Documentation
Configuration / SessionAuthenticator.user_key
This option enables you specify what session key to grab the user/login name from. The value of this session can then be substituted with the ${user} in any of the config parameters. Most common is to place it in the filesystem.rootpath config value, see below for a example. Notice a any \, / or : will be removed form the user name for security reasons.
$moxieManagerConfig['SessionAuthenticator.user_key'] = "UserName";
$moxieManagerConfig['filesystem.rootpath'] = 'users/${user}'; // Override root by username
<add key="SessionAuthenticator.user_key" value="UserName" />
<add key="filesystem.rootpath" value="UserName" />
Then you can set the session in your application and the variable in the rootpath will be replaced.
$_SESSION["UserName"] = "Tony";
Session["UserName"] = "Tony";