public class FtpSettingsSection : ConfigurationSection
{
    [ConfigurationProperty("host", IsRequired = true)]
    public HostElement Host
    {
        get { return (HostElement)this["host"]; }
        set { this["host"] = value; }
    }

}

public class HostElement : ConfigurationElement
{
    [ConfigurationProperty("URL", IsRequired = true)]
    public string URL
    {
        get { return (string)this["URL"]; }
        set { this["URL"] = value; }
    }
}