N-VOC 殿下（对除王、后以外王室成员的尊称） Expressions such as 'Your Highness' or 'His Highness' are used to address or refer to a member of the royal family other than a king or queen. 殿下（对除王、后以外王室成员的尊称）
That would be best, Your Highness.
那样最好了，殿下。
Her Royal Highness the Duchess of Kent opened an art exhibition recently.