What is the Congiguration manager class in the code and how to you set it in the Project or IDE?

How do I set AppSettings in an ASPX project's Configuration manager?

This is an ASPX/CS Project with Visual Studio 2010. It is a Configuration Manager question.

I am successfully debugging (sort of) some code that is being used already on a server. But there is a piece of code that plays with the URL in the live version that should not be used in the debug/localhost version.

Code:
     protected void Page_Load(object sender, EventArgs e)
        {
            if (ConfigurationManager.AppSettings["IsTesting"] == "false" && Request.Url.ToString().Contains("http:"))
            {
                Response.Redirect(Request.Url.ToString().Replace("http:", "https:"));
            }

            LoadMasterTemplate();
        }
This piece of code lands on the "Response.Redirect...." line when it should not because the "IsTesting" app setting should be set to true in the ConfigurationManager . How do I set that?