Execute code if string value in not null using C# in ASP.Net

here, i have to give url value for querystring only at the time of runtime. if i run this code now, here null reference of url is happening. so i dont want to run flow of code without im giving url value in url location area when the program running in a browser. guide me how to run program after url gets value from url location by manually giving querystring value like "?url=example.com".

var url=Request.QueryString["url"];
// Creates an HttpWebRequest with the specified URL.
HttpWebRequest myWebRequest = (HttpWebRequest)WebRequest.Create(url);

I want to run my code only after I give url value.