try
<script>
var mynew = null;
function opennew()
{
if (mynew==null || mynew.closed)
{
mynew=window.open('zip.html','newwindow','');
setTimeout("checkstatus()",1000);
}
}function checkstatus()
{
   if(mynew.document.readyState=='complete')
   {
mynew.document.all.mytext.value="welcome";
   }
   else
setTimeout("checkstatus()",1000);
}
</script><input type="button" value="open" onclick="opennew()">