If you want to get a specific user’s attention, the best way to do that is usually with a “mention”, a concept popularized by social media sites like Twitter.
To mention a user simply put a @ character in front of their name, like @Doc. Your post will then appear as a notification for that user so they will be alerted when next they visit the forum.
You can also type @ character followed by the first letters of that someone’s username to get a list of matching users. If you’re posting in an existing topic the forum will intelligently push active topic participants to the top of the auto-completion list.
Here is an example (it’s a video, click play at the bottom):