Storage Spam to Junk Folder

[fede]

Hi, my name is Fede, i`m from Argentina. Sorry for my spanish.
I a mail server with SpamAssassin and RoundCube. I change spam message subject 
with "***SPAM***", and in my config (main.inc.php) file of Roundcube i have 
this line:
--------------------------------------------
// store spam messages in this mailbox
$rcmail_config['junk_mbox'] = 'Junk';
--------------------------------------

But spam messages don´t save in Junk folder.
How I storage spam messages in Junk folder?

Very Thanks :D