'Make the secret from the form initial data and slip it into the form.'

def __init__(self, *args, **kwargs):

    from paypal.standard.helpers import make_secret
    super(PayPalSharedSecretEncryptedPaymentsForm, self).__init__(*args, **kwargs)
    secret_param = ('?secret=%s' % make_secret(self))
    if ('notify_url' in self.initial):
        self.initial['notify_url'] += secret_param
    else:
        self.fields['notify_url'].initial += secret_param
