It already "knows"... That's what SYSTEM_USER returns. For example...
SELECT SYSTEM_USER AS WindowsAccount, be.*
FROM Bakery.dbo.BakeEvents be;
I know, looks really stupid, but the first column is the user running the query or the account it's being run under.
Maybe you want to dump it into a conditional split sort of thing where if it's "User A", do one thing, and if it's "User B" do something else.