Yes you can put raw beef into a slow cooker and the slow cooker will cook the raw beef slowly.
The slow cooker is designed to cook everything that you place into the slow cooker including the raw beef and as long as you allow enough time for the slow cooker to slowly cook the beef it will fully cook and be ready and safe to eat.
For example if you put some raw beef into your slow cooker and go to work for 8 or 9 hours then the beef should be fully cooked after you return home as long as the slow cooker was on the highest setting.
The same goes for if you put raw beef into the slow cooker and then go to sleep which is also safe to do and I do it all the time.
By the time you wake up the beef should be fully cooked but you can also just turn the slow cooker down to the keep warm setting and then go to work or wait until dinner or lunch to eat the slow cooked beef.
You can brown the beef first for a richer flavor and the beef will slow cook much faster but putting the raw beef into the slow cooker without browning it or cooking beforehand is okay.