So what is Article 104 of the Basic Law of the Hong Kong Special Administrative Region?

Well it stipulates that "when assuming office, the Chief Executive, principal officials, members of the Executive Council and of the Legislative Council, judges of the courts at all levels and other members of the judiciary in the Hong Kong Special Administrative Region must, in accordance with law, swear to uphold the Basic Law of the Hong Kong Special Administrative Region of the People's Republic of China and swear allegiance to the Hong Kong Special Administrative Region of the People's Republic of China."