According to the Microsoft Documentation at: https://learn.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.http.querystring.add?view=aspnetcore-9.0

Request.Querystring.Add returns a QueryString, not Void. However, Request.QueryString.Add does not mutate the Request Query String, therefore, you need to do something like this:

Request.QueryString = Request.QueryString.Add("returnUrl", url.ToString());