<form id="myform" action="http://ymlp.com/subscribe.php?id=<?= $userid; ?>" type="POST">
    <input type="hidden" id="userid" name="userid" value="<?= $userid; ?>">
    <input type="submit" name="submit" id="submit" value="submit">
</form>
<script
  src="https://code.jquery.com/jquery-3.3.1.min.js"
  integrity="sha256-FgpCb/KJQlLNfOu91ta32o/NMZxltwRo8QtmkMRdAu8="
  crossorigin="anonymous"></script>
<script>
    $('#submit').on('click', function(e){
        e.preventDefault();
        var userid = $('#userid').val();
        $.ajax({
            type: "POST",
            url: "myphp.php",
            data: {'userid' : userid},
            success: function(data) {
                if ($.trim(data) == 'success') {
                    $('#myform').submit();
                }
            }
        });
    });
</script>