Should beef be cooked before adding to the pot for Stroganoff?
The recipe does not indicate that the beef should be browned before cooking and there is no need to. Per the instructions, all ingredients except the cream cheese go into the slow cooker together at the start. If it was intended for you to brown the beef first, that instruction would have been included.

The long, slow cooking should ensure that the meat will be tender and flavorful. Additionally, as in the picture of the dish, the beef cubes for stroganoff are on the small side. That should be additional insurance that your beef will be tender.