If you want to handle it on client side script Javascript/Jquery can be used .
You can try something like below
<script type="text/javascript">
function Addition() {
var TA = document.getElementById("TextBoxTA").value;
var TL = document.getElementById("TextBoxTL").value;
if ((TA != "") && (TL != "") && (typeof TA !== "undefined") && (TL !== "undefined")) {
var AddResult = parseInt(TA) + parseInt(TL);
document.getElementById("TextBoxTNA").value = AddResult.toString();
}
}
</script></script>
Call the JS script on any of the Keyup,Textchanged, Onblur events
<asp:TextBox ID="TextBoxTA" runat="server" onblur="Addition()" ></asp:TextBox>
<asp:TextBox ID="TextBoxTL" runat="server" onblur="Addition()"></asp:TextBox>
<asp:TextBox ID="TextBoxTNA" runat="server" ></asp:TextBox>
Hope this helps