Thats not something Roundcube itself can do. Roundcube is not moving the messages to the junk folder, I guess that being done my some filtering you have setup. Probably what you need to do is adjust the filtering rule so that it marks the message as read before it moves the message to the junk folder.
Some setups have plugins which let you manage filters from inside Roundcude, look for a Filters tab or similar on the Settings screen.
