ASP.Net add item to listbox without posting - asp.net

Question

I am trying to add items to a listbox. I have a textbox and a button and when the user enters something into the textbox and hits add, I want to insert a new item the listbox without the page refreshing.
Here is what I have so far:
<asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<div class="col-xs-6 text-center">
<asp:Label ID="empid" runat="server" Text="Employee ID" CssClass="label-default label "></asp:Label>
<asp:TextBox ID="txtEmployeeID" runat="server" CssClass="form-control"></asp:TextBox>
<div class="col-xs-6">
<asp:Button ID="btnAddEmp" runat="server" Text="Add Emp" CssClass="btn btn-primary top-buffer" Width="100%" />
</div>
<div class="col-xs-6">
<asp:Button ID="btnRemoveEmp" runat="server" Text="Remove Emp" CssClass="btn btn-danger top-buffer" Width="100%" />
</div>
<asp:ListBox ID="listEmps" runat="server"
CssClass="top-buffer add-height"
Width="100%"
ValidationGroup="req"></asp:ListBox>
</div>
</ContentTemplate>
</asp:UpdatePanel>
and here is my button click event
Protected Sub btnAddEmp_Click(sender As Object, e As EventArgs) Handles btnAddEmp.Click
If Not IsPostBack() Then
If listEmps.Items.Count = 0 Then
listEmps.Items.Add(txtEmployeeID.Text)
Else
'check list to see if employee already exists
For i = listEmps.Items.Count - 1 To 0 Step -1
If RTrim(txtEmployeeID.Text) = RTrim(listEmps.Items(i).ToString) Then
'employee already exists within list
Response.Write("<script>alert('Employee ID " & txtEmployeeID.Text.Trim & " has already been added. ')</script>")
txtEmployeeID.Text = ""
Exit Sub
End If
Next
'add to list
listEmps.Items.Add(txtEmployeeID.Text)
End If
End If
End Sub
How can I properly add items to a listbox without refreshing the page?

Answer 1

Remove your If Not IsPostBack() Then check in your button click event. The IsPostBack() is typically only used for the Page_Load event. Since your button is contained in an UpdatePanel the click event should be asynchronous and won't cause a postback.