How can I set application settings at install time (via installer class)

MessageBox.Show(Properties.Settings.Default.MySetting);

        Properties.Settings.Default.MySetting = "Foo";
        Properties.Settings.Default.Save();

        MessageBox.Show(Properties.Settings.Default.MySetting);

<appSettings file="user.config">
    <add key="foo" value="some value unchanged by setup"/>
</appSettings>

using System.Collections.Generic;
using System.Text;
using System.Xml;

namespace Utils
{
    public class ConfigGenerator
    {
        public static void WriteExternalAppConfig(string configFilePath, IDictionary<string, string> userConfiguration)
        {
            using (XmlTextWriter xw = new XmlTextWriter(configFilePath, Encoding.UTF8))
            {
                xw.Formatting = Formatting.Indented;
                xw.Indentation = 4;
                xw.WriteStartDocument();
                xw.WriteStartElement("appSettings");

                foreach (KeyValuePair<string, string> pair in userConfiguration)
                {
                    xw.WriteStartElement("add");
                    xw.WriteAttributeString("key", pair.Key);
                    xw.WriteAttributeString("value", pair.Value);
                    xw.WriteEndElement();
                }

                xw.WriteEndElement();
                xw.WriteEndDocument();
            }
        }
    }
}

string configFilePath = string.Format("{0}{1}User.config", targetDir, Path.DirectorySeparatorChar);

IDictionary<string, string> userConfiguration = new Dictionary<string, string>();

userConfiguration["Server"] = Context.Parameters["Server"];
userConfiguration["Port"] = Context.Parameters["Port"];

ConfigGenerator.WriteExternalAppConfig(configFilePath, userConfiguration);