Planet CakePHP

I am creating an application which tracks signatures form various organizations. I have a simple form which I can pass an ID to, so it will automatically select the right organization. The URL for add looks like this:

/signatures/add/3

The form works well. By passing 3, or any other ID, it automatically selects the right field, because in my view I do:

echo $form->input('organization_id', array('selected' => $this->passedArgs));

I run into my problem when the user forgets to fill out a form element. The form returns the user to:

/signatures/add/

So it doesn't have the right organization selected. It reverts to the default which is 1. Any tips on how I can retain my parameters?