Can someone please tell me what key i have to push or what i have to select on left for dead PC to be able to chat with the people im playing with on Campaign Mode online

To chat with all people in the server, press the "Y" button, type in your message, and press "Enter" or "Return". To chat with your team only (survivors or infected), press the "U" button, type in your message, and press "Enter" or "Return".