src/vs/workbench/browser/actions/workspaceCommands.ts/selectWorkspaceFolders
async function selectWorkspaceFolders(accessor: ServicesAccessor): Promise<URI[] | undefined> {
	const dialogsService = accessor.get(IFileDialogService);
	const pathService = accessor.get(IPathService);

	const folders = await dialogsService.showOpenDialog({
		openLabel: mnemonicButtonLabel(localize({ key: 'add', comment: ['&& denotes a mnemonic'] }, "&&Add")),
		title: localize('addFolderToWorkspaceTitle', "Add Folder to Workspace"),
		canSelectFolders: true,
		canSelectMany: true,
		defaultUri: await dialogsService.defaultFolderPath(),
		availableFileSystems: [pathService.defaultUriScheme]
	});

	return folders;
}