In general, codes of ethics are cited and referenced like reports published on a website, though they may also have an edition number in some cases. They usually have an organisation as the author.
Author(s). (Year). Title (edition if not first). URL
British Psychological Society. (2014). BPS Code of Human Research Ethics (2nd ed.). https://www.bps.org.uk/news-and-policy/bps-code-human-research-ethics-2nd-edition-2014
(British Psychological Society, 2014) or British Psychology Society (2014).
COSCA (Counselling and Psychotherapy in Scotland). (2014). COSCA Statement of Ethics and Code of Practice. http://www.cosca.org.uk/download_file/view/206/261
(COSCA, 2014) or COSCA (2014)…