To resolve your problem follow below step.
step 1: Create java script function
function txtOnkeypress()
{
// apply your logic here..
}
For example:
<script type="text/javascript">
function txtOnKeyPress(txt1)
{
if (txt1 != 'undefined')
{
txt2 = document.getElementById('<%=TextBox2.ClientId %>');
txt2.value = txt1.value;
}
}
</script>
step 2: call javascript function on textbox key press event.
onkeyPress = txtOnkeypress()
For example:
<asp:TextBox ID="texbox1" runat="server" onkeydown="txtOnKeyPress(this);" ></asp:TextBox>
<asp:TextBox ID="TextBox2" runat="server" ></asp:TextBox>
Thanks
Vipul patel