// Get tempstore. Within classes use dependency injection instead.
$tempstore = Drupal::service('user.private_tempstore')->get('mymodule_name');
// Set my_variable_name.
$tempstore->set('my_variable_name', $data);

// Get tempstore. Within classes use dependency injection instead.
$tempstore = Drupal::service('user.private_tempstore')->get('mymodule_name');
// Get my_variable_name data.
$data = $tempstore->get('my_variable_name');