Пример кода:
<html>
<head>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.2.2/jquery.min.js"></script>>
</head>
<body>
<script>
$("#button").click(function() {
$.ajax({
method: "POST",
url: "/send_data_2",
data: $('#comment').serialize(),
});
})
</script>
<form action="/send_data_1" method="post">
<fieldset>
<legend>Контактная информация</legend>
<p><label for="email">E-mail </label><input type="email" id="email"></p>
</fieldset>
<p><input id="button" type="submit" value="Отправить"></p>
</form>
<form action="/send_data_2" method="post">
<input type="hidden" name="comment" id="comment" value="12345">
</form>
</body>
</html>
