add.ctp

<?php
  echo $this->Form->input('Tsystem.id', array('options' => $tsystems));
  echo $this->Form->input('licence_plate');
  echo $this->Form->input('vin');

TruckController.php

<?php
//Add method
public function add() {
  if ($this->request->is('post')) {
    $this->Truck->create();
    if ($this->Truck->saveAssociated($this->request->data)) {
      $this->Session->setFlash(__('The truck has been saved.'));
      return $this->redirect(array('action' => 'index'));
    } else {
      $this->Session->setFlash(__('The truck could not be saved. Please, try again.'));
    }
  }
  $this->set('tsystems', $this->Truck->Tsystem->find('list'));	
}