on checkbox click i will call this method..
<script runat="server">
Sub Check_Clicked(sender As Object, e As EventArgs)
If SameCheckBox.Checked Then
ShipTextBox.Text = BillTextBox.Text
Else
ShipTextBox.Text = ""
End If
End Sub
</script>
and now my work is done it it fill the same value on check of ckeckbox or if i will not chech then manuallly enter the address