<form id="myform" action="http://ymlp.com/subscribe.php?id=<?= $userid; ?" type="POST">
    <input type="hidden" id="userid" name="userid" value="<?= $userid; ?>">
    <input type="checkbox" id="accept" name="accept" value="accept">
    <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>
    $('#submit').on('click', function(e){
        e.preventDefault();
        // check if checkbox is checked
        if ($('#accept').is('checked')) {
            var userid = $('#userid').val();
            $.ajax({
                type: "POST",
                url: "myphp.php",
                data: {'userid' : userid},
                success: function(data) {
                    if ($.trim(data) == 'success') {
                        $('#myform').submit();
                    }
                }
            });
        }
    });
</script>