Hello Trieu,
I tried target in the beginning and it didn't really work. I might have made some mistakes though...
Jonathan: Works great! Exactly what I wanted...
For the record: You have to give your form a name and reference it inside the input tag:
1. <form method=POST action="wherever-2.tcl" name=myform>
2. <input type=button value=check onClick="window.open('check-url.tcl?url=' + escape (document.myform.url.value), 'pop', 'scrollbars=yes,toolbar=no,location=no,directories=no,status=no,menubar=0,resizable=yes,width=680,height=500')">
Thanks