I figured it out. It you type @ and then the userid of the person, it will work. The trouble is I was assuming the userid is the first initial and the last name. Most people have their userid as the full first name, a dash, and then their full last name. You can guess at it, but the text editor will not tell you if you are right until you post it, unless the name is on your friend list.
So what you can do is view the person’s profile. The userid is the @ sign following the person’s name at the top of the screen. I don’t know how I missed that!