Adding javascript to htmltextwriter, how can I get the value from javascript before navigation

Adding javascript to htmltextwriter

RenderJavascript(HtmlTextWriter output) { output.WriteLine(" var newUrl =url+\"&month=\"+obj.value;"); output.WriteLine(" window.location = newUrl; "); }

How to assign newUrl value in to a string, before navigation

I am expecting result like below format

RenderJavascript(HtmlTextWriter output) { output.WriteLine(" var newUrl = url+\"&month=\"+obj.value;"); output.WriteLine(" window.location = newUrl; "); string resulturl = ...........; }