I am trying to cold mail an employee from an organization about interview preparation etc so I can get a job offer.
Which Stack Exchange site would be suitable for such discussion? Workplace?
More than likely The Workplace, but check their topic guidelines. Depending on the industry and type of job, you might be able to pick up more cues about the expectations surrounding correspondence of this nature on Academia, for instance.
I'd suggest a search on The Workplace first, because this topic has been covered a few times in general there, so you might be able to at least reference a relevant existing question if it doesn't quite cover the particulars that you need.
It wouldn't be out of line to ask about the prerequisites needed for someone to consider replying to cold correspondence in any relevant chat rooms, too, as it's an interesting question. A physicist and a software engineer might look for different things. Just be considerate and don't push it if you don't get a good reception.