Parent form :
var name = document.getElementById("<%= TextBox1.ClientID %>").value;
window.open("page2.aspx?id=" + name);
Child Form:
<script type="text/ecmascript">
function getQuerystring(key, default_) {
if (default_ == null) default_ = "";
key = key.replace(/[\[]/, "\\\[").replace(/[\]]/, "\\\]");
var regex = new RegExp("[\\?&]" + key + "=([^&#]*)");
var qs = regex.exec(window.location.href);
if (qs == null)
return default_;
else
return qs[1];
}
var id = getQuerystring('id');
document.getElementById("<%= TextBox1.ClientID %>").value = id;
</script>