lingers says:
data: "{'regno':'" + document.getElementById('txtSearch').value + "'}",
There is no need to change the parameter value. Only you need to change the parameter name as per your WebMethod parameter name.
data: "{'regno':'" + request.term + "'}",
Refer the below updated code.
HTML
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>jQuery Show Records Found Message in AutoComplete</title>
    <link href="http://code.jquery.com/ui/1.11.4/themes/ui-lightness/jquery-ui.css" rel="stylesheet" type="text/css" />
    <script type="text/javascript" src="http://code.jquery.com/jquery-1.10.2.js"></script>
    <script type="text/javascript" src="http://code.jquery.com/ui/1.11.4/jquery-ui.js"></script>
    <script type="text/javascript">
        $(function () {
            SearchText();
        });
        function SearchText() {
            $(".autosuggest").autocomplete({
                source: function (request, response) {
                    $.ajax({
                        type: "POST",
                        contentType: "application/json; charset=utf-8",
                        url: "Default.aspx/GetAutoCompleteData",
                        data: "{'regno':'" + request.term + "'}",
                        dataType: "json",
                        success: function (data) {
                            if (data.d.length > 0) {
                                response($.map(data.d, function (item) {
                                    return {
                                        label: item.split('/')[0],
                                        val: item.split('/')[1]
                                    }
                                }));
                            }
                            else {
                                response([{ label: 'No Records Found', val: -1 }]);
                            }
                        },
                        error: function (result) {
                            alert("Error");
                        }
                    });
                },
                select: function (event, ui) {
                    if (ui.item.val == -1) {
                        return false;
                    }
                    $('#lblUserId').text(ui.item.val);
                }
            });
        }
    </script>
</head>
<body>
    <form id="form1" runat="server">
        <div class="demo">
            <asp:HiddenField ID="hdnId" runat="server" />
            <div class="ui-widget">
                <label for="tbAuto">Enter UserName: </label>
                <input type="text" id="txtSearch" class="autosuggest" />
            </div>
            <div> </div>
            <div>
                Selected UserId:<b><label id="lblUserId" /></b>
            </div>
        </div>
    </form>
</body>
</html>
Code
[WebMethod]
public static List<string> GetAutoCompleteData(string regno)
{
    List<string> result = new List<string>();
    using (SqlConnection con = new SqlConnection("Data Source=NER\\SQLEXPRESS01;Integrated Security=true;Initial Catalog=kaging;"))
    {
        using (SqlCommand cmd = new SqlCommand("select id,regno from Account where regno LIKE '%'+@SearchText+'%'", con))
        {
            con.Open();
            cmd.Parameters.AddWithValue("@SearchText", regno);
            SqlDataReader dr = cmd.ExecuteReader();
            while (dr.Read())
            {
                result.Add(string.Format("{0}/{1}", dr["regno"], dr["id"]));
            }
            return result;
        }
    }
}
Screenshot
