Copied value from one textbox to another textbox not saving in button click using JavaScript in ASP.Net

<script language="javascript" type="text/javascript">

function copy() {

        var instrasstno = document.getElementById("<%=Txtbox1.ClientID%>").value;     
        

        document.getElementById("<%=TxtCmpntAssetNo.ClientID %>").value = instrasstno ;
    }

</script>




<table>
<tr>
<td><label class="Lbl">1st Textbox</label></td>
<td>
<asp:TextBox ID="Txtbox1" CssClass="MndtryTxtBox" runat="server" Width="70px" onkeyup="copy();"></asp:TextBox></td>
<td><label class="Lbl">2nd Textbox</label></td>
<td><asp:TextBox ID="TxtCmpntAssetNo" runat="server" CssClass="MndtryTxtBox" Width="100px" Enabled="false"></asp:TextBox></td>

<td><asp:Button ID="Btn" Text="ADD" runat="server" onclick="Btn_Click"/></td>

</tr>
</table>

Hi all, by using above code i am copying value from one textbox to another textbox. its working good but save button click 2nd textbox value showing empty but in 2nd textbox i have value, but i want save the second textbox value also.. if any body understand my problem plz help me

Thanks in advance