Save config after

SaveConfigurationAfter.cs

var ConfigurationFileMap = new ExeConfigurationFileMap() { ExeConfigFilename = "My.config" };
var config = ConfigurationManager.OpenMappedExeConfiguration(ConfigurationFileMap, ConfigurationUserLevel.None);
if (config != null)
{
    // do stuff
    config.Save();
    
    // Format XML
    XDocument configXml = XDocument.Load(configuration.FilePath);
    configXml.Save(configuration.FilePath, SaveOptions.None);
}