How do I pre-key footage in After Effects?

[Harrison Sheen]

An example of what i'm talking about is the action movie essentials 2 off video copilot. If you drop any movie file from action essentials 2 over the top of some other footage, the black background is already transparent. Does anyone know how to pre-key some footage of something like smoke against black, so that I can keep the black background transparent on any movie clip that I drop the smoke footage onto?

Does that make sense?

Thanks