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.

Example: British Psychological Society’s Code of Human Research Ethics.

Reference:

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

Citation:

(British Psychological Society, 2014) or British Psychology Society (2014)...

Example: COSCA’s Statement of Ethics and Code of Practice.

Reference:

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

Citation:

(COSCA, 2014) or COSCA (2014)…