I write a post at asp.net forum and I get 4 suggestion in the first hour, not so here.
So, my solution has been write it to a file:
' Delete the file first
Using _fs As System.IO.FileStream = New FileStream(_fileName, FileMode.CreateNew)
var app = '<%= Request.ApplicationPath %>';
if (app == '/') app = '';
var aspx = app + formPath;
window.open(aspx, windowName, 'scrollbars=yes,resizable=yes,menubar=yes,toolbar=yes,width=800,height=550', false);