使用Button（CakePHP）从另一个模型添加Join

I have a Course model and a User model joined by Subscription. I want to create a postLink form helper on the Courses index page that would automatically subscribe the user to that, but I don't know how to pass the parameters correctly. In essence, I want the user to click Subscribe, and have a subscriptions/add form automatically submitted so that the join table has another record.

My specific question is: How do I use a model in another model's view?

Here's the Courses index.ctp array from print_r:

Array
(
    [0] => Array
        (
            [Course] => Array
                (
                    [id] => 1
                    [name] => Flying
                    [created] => 2014-01-27 19:05:43
                    [modified] => 2014-01-27 19:05:43
                )

            [Subscription] => Array
                (
                    [0] => Array
                        (
                            [id] => 2
                            [user_id] => 2
                            [course_id] => 1
                        )

                )

        )
)

Here's my bad postLink in the Course index.ctp view:

<?php 

  echo $this->Form->postLink(__('Subscribe'),
                            array( 'controller' => 'Subscriptions', 
                                   'action'     => 'add'            ), 
                            null, 
                            __('Are you sure you want to subscribe to # %s?', 
                            $course['Course']['name'])); 
?>

Assuming they're associated, you can do like you would in the Controller - just don't include the model you're actually in, since $this is already that model:

$this->AssociatedModel->save($data);