how can i set App.Config values while testing with nunit?

how can i set values in order to test my code.

   public Hashtable MyMethod()
        {
            string _pathFTP = ConfigurationSettings.AppSettings["FTP_HOST_FEEDING_CONTAINER"];
            string _user = ConfigurationSettings.AppSettings["FTP_USER"];
            string _pass = ConfigurationSettings.AppSettings["FTP_PASS"];

          ...
         }


  now , i'd like to test this with Nunit,
i have a test method as follow.

 [Test]
public void testing MyMethod(){
                      <---- how can i set my ftp values, as they are not existent in my method process while testing
    assert(...);
}

the aim is not to change my method code,