test.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Flask-SocketIO Client</title>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/socket.io/4.1.2/socket.io.js" integrity="sha384-EwQIfJBLgjys3w8Xb8ZDPYtJkmA8CyUD4f88N4CgEMhuMquuOmxX5iC6QloS4h9W" crossorigin="anonymous"></script>
    <script type="text/javascript">
        var socket = io.connect('http://' + document.domain + ':' + location.port);

        function sendMessage() {
            var message = document.getElementById('message_input').value;
            socket.emit('message', message);
        }
    </script>
</head>
<body>
    <input type="text" id="message_input" />
    <button onclick="sendMessage()">Send</button>
</body>
</html>