How to add a black censor bar to a face that moves with the face?

Does it have to be a black censor bar? if not look at the Censor effect.

If it has to be a black bar. Make the bar using a Custom generator in Solids. Crop it to whatever size you want using the Crop function in the Viewer. Use the Transform function in the Viewer to position. You can keframe the crop and transform functions to animate the bar as needed.