I have a new field in customer register form named referral_code i just want to get this field data in my UpdateCustomerMeta observer how can i get the value of the field "referral_code" in observer of customer_register_success
In that event you can access controller (and so, any parameter submitted)
$this->_eventManager->dispatch( 'customer_register_success', ['account_controller' => $this, 'customer' => $customer] );
So I think you should be able to get the request parameters this way...
$parameters = $observer->getEvent()->getAccountController()->getRequest()->getParams();