> That's not my request, I only want a command or a keybinding.

You could try to bind

(defun my-cpp-comment-region (beg end &optional arg)
   (interactive "*r\nP")
   (let ((comment-start "/* ")
	(comment-end " */"))
     (comment-region beg end arg)))

to a key.

BTW with comment-styles box / box-multi and newline terminated comment
syntax `comment-region' comments out the entire text from the beginning
of the region till EOL.  The attached patch should cure that.