Widnow popup

Hi , I am trying to create a popup window. I put down the below code but i dont see window popup. 
This code resides in side a button click event. I want to open the popup window based on a condition that is why I put this code inside the button even.
string script =
                        @" window.open('DupesMembers.aspx', 'pop_window', 'toolbar=no,menubar=no,location=no,scrollbars=yes,height=300,width=500,left=400,top=200');";
                    
                   Page.ClientScript.RegisterStartupScript(this.GetType(),"Script", script,true);

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