It can be either!

* If the test involves milk, then it is added to the bowl before pouring the infused tea into it, unless that is contrary to the organisation's normal practice.

* If milk is added after the pouring of tea, the standard notes that best results are obtained when the liquid is between 65 and 80 °C

My organization tends to prefer milk after, But I'm a silly yank who has lots of wrong opinions on a great deal of things (or so I've been told).