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;