how to retrieve additional fields on the edit product page?

I have added an additional field to the admin edit product page with this code in my custom plugin

Code:

// this method will be called in the product edit page
    function onProductForm(Am_Event $event){
        $form = $event->getForm();

        $form->addElement('text', 'ab_return_url', array('size' => 60, 'class'=>'translate'))
                ->setLabel(___("Paypal Return URL\n" .
                "This is the url the thanks page will redirect to after payment\n" .
                "You can use this for one time offers"));

    }

and it adds it to the 'Additional' field set but the value is not saving.

would you tell me what code I would need to use to add the field and it keeps its value when saved and also what code I can use to retrieve that value on the onThanksPage event?

[update]

Ok I realized that it's not saving because there isn't a field available on the am_billing_plans table where everything else gets saved.

I am instead going to use a custom brick so I can add the redirect url to the signup form for a product.
the brick is added and it can be configured ok and the data gets saved.

I am using this code

Code:

class Am_Form_Brick_redirectUrl extends Am_Form_Brick
{
    protected $hideIfLoggedInPossible = self::HIDE_DONT;

    public function __construct($id = null, $config = null)
    {
        $this->name = ___('Thanks Redirect URL (Hidden)');
        parent::__construct($id, $config);
    }
    public function initConfigForm(Am_Form $form)
    {
        $form->addText('redirect_url')->setLabel(___('Redirect URL'));
    }

    public function insertBrick(HTML_QuickForm2_Container $form)
    {
        $form->addHidden('redirect_url')->setValue($this->getConfig('redirect_url'));
    }

}

Will this data be available in the onThanksPage event?

ie. Can the signup form that was used for the purchase be retrieved so I can setRedirect to use the URL that was saved to the custom brick?
eg;

Code:

// this method will be called before rendering "thanks" page
    function onThanksPage(Am_Event $event)
    {
        $controller = $event->getController(); // ThanksController instance
        $invoice = $event->getInvoice(); // Invoice instance OR NULL (!)
        if (!$invoice) return;

         // get signup form here
         // get value saved in redirect_url
         $savedredirecturl = (how to get it?)
         $controller->getResponse()->setRedirect($savedredirecturl);

    }

ah of course I can use the $_SESSION global to store that because both the form and the payment would always be in the same session. I was over thinking it. thanks Alexander

sorry Alexander but I'm having trouble with getting the parameters . I keep getting a 'cannot access protected property' with this code in my custom amember plugin

Code:

function onSignupUserUpdated (Am_Event $event){
        $vars = $event->vars;
}

can you give me the code to access the $vars ?

sorry Alexander, I found it.

Code:

$vars = $event->getvars();

I'm just getting used to the new classes but I'm getting there!