I set up an external domain so that it points to my Netlify site using DNS at orsem2020.netlify.app to use admuorsem2020.com.
Even though my primary domain (admuorsem2020.com) has an SSL certificate, the subdomain I set up (with an A record) in api.admuorsem2020.com does not have one, which I need for the client server to get API requests from.