If you want to do that you need to create a 4:3 project, insert your widescreen video so you have the space at the top and bottom, then put in your text. The result will not be Widescreen but will simulate letterbox in 4:3
If you created a widescreen video and you see black bars you are watching it on a standard monitor. You will not see the bars at the top and bottom if you watch a widescreen video on a widescreen monitor/tv.
