When in the Zoom meeting, if you can’t see the chat window, when hovering over the main video, you should see a button at the bottom of the page that says ‘Chat’. Click this and the chat window will appear.
From within this newly opened window, at the bottom it will say ‘Type message here…’. Just above this field, it will say ‘To:’, and then say ‘Everybody’. This means that everybody in the chat window will see your message. If it does not say everybody, you can click on it and then select ‘Everybody’ from the list, or you can select another person to send a private message to. Then click where it says ‘Type message here…’, type your message and then hit enter.
(Image showing where the ‘Chat’ icon will appear)
(Image showing the text chat, which should appear in the bottom right of your screen)