Where is the best place to ask the user their ethnicity?

I'm building an enterprise mobile application for health and medical purposes, and due to the data driven nature of the app we ask users to enter their ethnicity. It's entirely optional but it allows us to better tune our analysis which translates to better results for them. I was thinking about asking on the signup page, but perhaps this is not the right place to ask.

What is the standard list and selection method? Should it be single or multi-select? Is there a standard comprehensive list of ethnicities that includes everyone? I've looked at various other websites but either this field isn't present or where it is the list is different.