You can achieve this functionality in numerous ways with both Server Side code (C#) or with Client Side code (JavaScript or jQuery). Without seeing what you currently have built, it is hard to tell you what the best fit is... Below is a sample of how to populate a text box with data from another with the Check Box is checked...
SERVER SIDE EXAMPLE
C#
protected void MyCheckBox_CheckedChanged(object sender, EventArgs e)
{
this.MySecondTextBox.Text = this.MyFirstTextBox.Text;
}
ASP
<asp:TextBox ID="MyFirstTextBox" runat="server"></asp:TextBox>
<asp:CheckBox ID="MyCheckBox" runat="server"
oncheckedchanged="MyCheckBox_CheckedChanged" />
<asp:TextBox ID="MySecondTextBox" runat="server"></asp:TextBox>
CLIENT SIDE EXAMPLE
C#
protected void Page_Load(object sender, EventArgs e)
{
this.MyCheckBox.Attributes.Add("onClick", "CopyText()");
}
ASP
<asp:TextBox ID="MyFirstTextBox" runat="server"></asp:TextBox>
<asp:CheckBox ID="MyCheckBox" runat="server" />
<asp:TextBox ID="MySecondTextBox" runat="server"></asp:TextBox>
<script type="text/javascript" language="javascript">
function CopyText() {
var txt2 = document.getElementById("<%= this.MySecondTextBox.ClientID %>");
txt2.value = document.getElementById("<%= this.MyFirstTextBox.ClientID %>").value;
}
</script>