public string CustomDefaultToolbox {get; set;}

{
  var startMode = ApplicationOptions.GeneralOptions.StartupOption;
  var aprx_path = ApplicationOptions.GeneralOptions.StartupProjectPath;

  var hf_option = ApplicationOptions.GeneralOptions.HomeFolderOption;
  var folder = ApplicationOptions.GeneralOptions.CustomHomeFolder;

  var gdb_option = ApplicationOptions.GeneralOptions.DefaultGeodatabaseOption;
  var def_gdb = ApplicationOptions.GeneralOptions.CustomDefaultGeodatabase;

  var tbx_option = ApplicationOptions.GeneralOptions.DefaultToolboxOption;
  var def_tbx = ApplicationOptions.GeneralOptions.CustomDefaultToolbox;

  var create_in_folder = ApplicationOptions.GeneralOptions.ProjectCreateInFolder;
}

{
  //Set the application to use a custom project, home folder, gdb, and toolbox
  //In each case, the custom _path_ must be set _first_ before 
  //setting the "option". This ensures the application remains 
  //in a consistent state. This is the same behavior as on the Pro UI.
  if (string.IsNullOrEmpty(ApplicationOptions.GeneralOptions.StartupProjectPath))
    ApplicationOptions.GeneralOptions.StartupProjectPath = @"D:\data\usa.aprx";//custom project path first
  ApplicationOptions.GeneralOptions.StartupOption = StartProjectMode.WithDefaultProject;//option to use it second

  if (string.IsNullOrEmpty(ApplicationOptions.GeneralOptions.CustomHomeFolder))
    ApplicationOptions.GeneralOptions.CustomHomeFolder = @"D:\home_folder";//custom home folder first
  ApplicationOptions.GeneralOptions.HomeFolderOption = OptionSetting.UseCustom;//option to use it second

  if (string.IsNullOrEmpty(ApplicationOptions.GeneralOptions.CustomDefaultGeodatabase))
    ApplicationOptions.GeneralOptions.CustomDefaultGeodatabase = @"D:\data\usa.gdb";//custom gdb path first
  ApplicationOptions.GeneralOptions.DefaultGeodatabaseOption = OptionSetting.UseCustom;//option to use it second

  if (string.IsNullOrEmpty(ApplicationOptions.GeneralOptions.CustomDefaultToolbox))
    ApplicationOptions.GeneralOptions.CustomDefaultToolbox = @"D:\data\usa.tbx";//custom toolbox path first
  ApplicationOptions.GeneralOptions.DefaultToolboxOption = OptionSetting.UseCustom;//option to use it second
}

{
  //Default options can be set regardless of the value of the "companion"
  //path (to a project, folder, gdb, toolbox, etc.). The path value is ignored if
  //the option setting does not use it. This is the same behavior as on the Pro UI.
  ApplicationOptions.GeneralOptions.StartupOption = StartProjectMode.ShowStartPage;
  ApplicationOptions.GeneralOptions.HomeFolderOption = OptionSetting.UseDefault;
  ApplicationOptions.GeneralOptions.DefaultGeodatabaseOption = OptionSetting.UseDefault;
  ApplicationOptions.GeneralOptions.DefaultToolboxOption = OptionSetting.UseDefault;//set default option first

  //path values can (optionally) be set (back) to null if their 
  //"companion" option setting is the default option.
  if (ApplicationOptions.GeneralOptions.StartupOption != StartProjectMode.WithDefaultProject)
    ApplicationOptions.GeneralOptions.StartupProjectPath = null;
  if (ApplicationOptions.GeneralOptions.HomeFolderOption == OptionSetting.UseDefault)
    ApplicationOptions.GeneralOptions.CustomHomeFolder = null;
  if (ApplicationOptions.GeneralOptions.DefaultGeodatabaseOption == OptionSetting.UseDefault)
    ApplicationOptions.GeneralOptions.CustomDefaultGeodatabase = null;
  if (ApplicationOptions.GeneralOptions.DefaultToolboxOption == OptionSetting.UseDefault)
    ApplicationOptions.GeneralOptions.CustomDefaultToolbox = null;
}