how does a foreigner get a hong kong senior citizen id

All Hong Kong residents aged 65 or above can apply for a Senior Citizen Card. You should submit your application to the Senior Citizen Card Office in Room 2361, 23/F., Wu Chung House, 213 Queen's Road East, Wanchai, Hong Kong. The requirements are the application form, the copy of the ID Card and the photograph.
For more details, you can read http://www.swd.gov.hk/en/index/site_pubsvc/page_elderly/sub_csselderly/id_seniorciti/ as a reference.