TextBox With JQuery AutoComplete / Assign Selected ID Of Returned Data To An HiddenField?

i am using JQuery UI Autocomplete with asp.net textbox.AutoComplete works right.but how can i assign selected ID of returned Data to an hiddenField?My server Side Function returned list of objects that contains (this is an example) :

public List<Employee> GetEmployeeList()
{
List<Employee> empList = new List<Employee>();
empList.Add(new Employee() { ID = 1, Email = "Mary@somemail.com" });
empList.Add(new Employee() { ID = 2, Email = "John@somemail.com" });
empList.Add(new Employee() { ID = 3, Email = "Amber@somemail.com" });
empList.Add(new Employee() { ID = 4, Email = "Kathy@somemail.com" });
empList.Add(new Employee() { ID = 5, Email = "Lena@somemail.com" });
empList.Add(new Employee() { ID = 6, Email = "Susanne@somemail.com" });
empList.Add(new Employee() { ID = 7, Email = "Johnjim@somemail.com" });
empList.Add(new Employee() { ID = 8, Email = "Jonay@somemail.com" });
empList.Add(new Employee() { ID = 9, Email = "Robert@somemail.com" });
empList.Add(new Employee() { ID = 10, Email = "Krishna@somemail.com" });
return empList;
}
and this is ASPX Code :
<form id="form1" runat="server">
<div class="demo">
<div class="ui-widget">
<label for="tbAuto">
Enter Email:
</label>
<asp:TextBox ID="tbAuto" class="tb" runat="server">
</asp:TextBox>
</div>
</div>
<asp:TextBox ID="TextBox1" runat="server">
</asp:TextBox>
<asp:Label runat="server" ID="lbl" Text=""></asp:Label>
<asp:HiddenField runat="server" ID="hidid" />
<asp:Button ID="Button1" runat="server" Text="Button" />
</form>

here is my jquery Code :

<script type="text/javascript">
$(function () {
$(".tb").autocomplete({
select: function( event, ui ) {
// now assign the id of the selected element into your hidden field
$("#<%= hidid.ClientID %>").val( ui.item.ID );
},
source: function (request, response) {
$.ajax({
url: "Default.aspx/FetchEmailList",
data: "{ 'mail': '" + request.term + "' }",
dataType: "json",
type: "POST",
contentType: "application/json; charset=utf-8",
dataFilter: function (data) { return data; },
success: function (data) {
response($.map(data.d, function (item) {
return {
value: item.Email
}
}
)
)
}
,
error: function (XMLHttpRequest, textStatus, errorThrown) {
alert(textStatus);
alert(errorThrown);
}
});
},
minLength: 1
});
});
</script>

And this is My WEb Method Side Code :

<WebMethod()> _
Public Shared Function FetchEmailList(ByVal mail As String) As List(Of Employee)
Dim emp = New Employee()
Dim fetchEmail = emp.GetEmployeeList()
Return fetchEmail
End Function