Adding an ontextchanged event to a textbox within the signup component

i am trying to add a userna开发者_如何学Cme availability checker to the signup component and i used this :

    protected void checkUser(object sender, EventArgs e)//checking the user name avaliabilty
{ 
    if (Membership.GetUser(UsernameCheck.Text) != null)
    {
        usernameResult.Text = "User Name already Taken";
    }
    else 
    {
        usernameResult.Text = "User Name Avaliable";
    }
}

and this is my aspx :

  <tr>
                        <td align="right">
                            <asp:Label ID="UserNameLabel" runat="server" AssociatedControlID="UserName"></asp:Label>
                        </td>
                        <td>
                            <asp:TextBox ID="UserName" runat="server" BorderStyle="None" BorderWidth="1px" 
                                CssClass="SuserNameBox" AutoCompleteType="Disabled" AutoPostBack="true" OnTextChanged="checkUser" >Name Extension</asp:TextBox>
                            <asp:RequiredFieldValidator ID="UserNameRequired" runat="server" 
                                ControlToValidate="UserName" ErrorMessage="User Name is required." 
                                ToolTip="User Name is required." ValidationGroup="mainSignUp">*</asp:RequiredFieldValidator>
                        </td>

                    </tr>

but the ontextchanged event wont trigger (i tried it with an external text box and it worked fine) .

so my question is how to trigger an event to an element within the sign up component

using asp.net 3.5

thanks

<asp:UpdatePanel ID="UpdatePanel1" runat="server">
    <asp:TextBox ID="UserName" runat="server" BorderStyle="None" BorderWidth="1px"
                 CssClass="SuserNameBox" AutoCompleteType="Disabled" AutoPostBack="true" OnTextChanged="checkUser" >Name Extension</asp:TextBox>   
                 <Triggers>
                    <asp:AsyncPostBackTrigger ControlID="UserName" EventName="TextChanged" />
                 </Triggers>                         
</asp:UpdatePanel>

Something like this.