This function is enabled with a simple checkbox on the mail filter tab of the mailbox, the checkbox is labeled "Move Spam Emails to Junk directory.". I guess you don't need a tutorial for enabling a checkbox?
When I go to make the above change in Roundcube I get this "Soap Error: Could not connect to host". It's a Debian server with PHP 5.6.20. I've seen this problem before. Is it a Certificate issue? EDIT: Yes it was a certificate issue. I reinstalled ISPConfig and generated a new certificate and this resolved the issue.