asp.net c# textox_textchanged not firing requiredfieldvalidator on client side

Hello,

I have a textbox with a textbox_TextChanged event (empty at this time)

My *.aspx code is as follows:

 <asp:TextBox ID="txtGroupName" runat="server" Width="500" MaxLength="500" OnTextChanged="txtGroupName_TextChanged" AutoPostBack="True" CausesValidation="True" ValidationGroup="TCUserEntry" OnKeyDown="TextChanged(this)">


                </asp:TextBox>&nbsp;&nbsp;<span style="color: red">*</span>&nbsp; &nbsp;&nbsp;  
                     
                     <asp:RequiredFieldValidator ID="RFV_GroupName" ControlToValidate="txtGroupName" runat="server" ErrorMessage="Group Name is Required" Display="Dynamic" ValidationGroup="TCUserEntry" SetFocusOnError="true" TabIndex="0"></asp:RequiredFieldValidator>

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How do I trigger the required field validator control when the user tabs away from this control?

Thank you in advance.

Luis