register.html

<!DOCTYPE html>
<html>

<head>
	<meta charset="utf-8">
	<title>Registration</title>
	<script src='https://www.google.com/recaptcha/api.js'></script>
</head>

<body>
	<div>
		<h1>Registration form</h1>
		<form action="reg" method="POST">
			<input type="hidden" name="_csrf" id="_csrf">
			<input type="text" name="username" placeholder="Username" required>
			<input type="password" name="password" placeholder="Password" required>
			<input type="password" name="repeatpass" placeholder="Repeat Password" required>
			<input type="email" name="email" placeholder="Enter Email" required>
			<div class="g-recaptcha" data-sitekey="6LfoeK8ZAAAAAItlg0_RwUcMV0qLzCG4WX4j2xmL"></div><br>
			<input type="submit">
		</form>
	</div>
</body>
<script
  src="https://code.jquery.com/jquery-3.5.1.slim.min.js"
  integrity="sha256-4+XzXVhsDmqanXGHaHvgh1gMQKX40OUvDEBTu8JcmNs="
  crossorigin="anonymous"></script>
<script>
	$(document).ready(function () {
		$('#comment_form').submit(function () {
			$(this).ajaxSubmit({
				error: function (xhr) {
					status('Error: ' + xhr.status);
				},
				success: function (response) {
					console.log(response);
				}
			});
			return false;
		});
	});

	let url = "https://localhost:3000/token";
	fetch(url).then(response => response.json())
		.then((result) => {
			document.getElementById('_csrf').value = `${result.csrfToken}`;
		})
		.catch(error => console.log(error));
</script>

</html>