Hello! Ask away.

If you have a question, put it in your first message. For example:

Hey! I'm trying to get X working, but I get Y when I do Z. Any idea what's wrong?

No need to wait for a reply before asking. Most people check chat between other things, often from another time zone. If your question is already there, they can answer it when they see it. If all they see is "hello", they have to reply and wait for you, and by then you may have left.

Hello is fine. Hello plus your question is better.

Everyone has done it the other way at some point. No harm done.