place the code below in your code behind file from where you want to get the popup in another window.
string script = "";
script += "<script language='javascript' type='text/javascript'>";
script += " var win=window.open('~/PopUpPage.aspx', 'popup','width=802,height=500,directories=no,location=no,menubar=no,resizable=1,scrollbars=1,status=no,toolbar=no');";
script += "</script>";
ClientScript.RegisterClientScriptBlock(this.GetType(), "Test", script);
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