src/FolderDropdownField.php
public function setLastFolderID($folderID)
    {
        $request = Controller::curr()->getRequest();
        $session = $request->getSession();
        $session->set(get_class() . '.FolderID', $folderID);
    }
public function setLastFolderID($folderID)
    {
        $request = Controller::curr()->getRequest();
        $session = $request->getSession();
        $session->set(get_class() . '.FolderID', $folderID);
    }
Set the last folder selected

@param int $folderID Folder ID