Do you mean on this forum? It goes through user profiles (like https://discuss.dev.twitch.com/u/Liptonus777) and then a button labeled “Message” in the top right-hand corner when signed in.
If you’re looking for help on a specific subject, feel free to post about it. If you’re looking for private consultation or discussion, you’ll probably need to at least mention why you need help so that an applicable person can attempt to help.
If you’re looking for help on Twitch whispers, try searching the forum or the documentation for where to get started.