On, in or at?

Tweet

When I was starting my last discussion, an idea happened to me as well as a question. After we logged in, how can we descibe our statement? Can we say I am on mylot, in mylot or at mylot? That's may not a question for an Enlish native or whose mother language is English, but I am not one of that people, so I am a little confused and I wish someone can make me clear about that. Thanks in advance.