App.js

const [messageBody, setMessageBody] = useState('');

// useEffect(...) for loading initial messages

// useEffect(...) for subscriptions

const handleChange = (event) => {
  setMessageBody(event.target.value);
};

const handleSubmit = async (event) => {
  event.preventDefault();
  event.stopPropagation();

  const input = {
    channelID: '1',
    author: 'Dave',
    body: messageBody.trim()
  };

  try {
    setMessageBody('');
    await API.graphql(graphqlOperation(createMessage, { input }))
  } catch (error) {
    console.warn(error);
  }
};

// return (...)