Every time you eat any kind of cookies with tea, you had to dunk at least once and say “Good luck”.
I don’t know where it comes from, but dunking a cookie into tea is good luck. When eating a cookie with a beverage, it is bad luck not to dunk at least once. My mother would say she was “dunking for good luck” and that her father did it too. But I only remember it with tea.
Her father was English so it might have had something to do with the tea. Because tea and the tea industry is supposed to have such positive connotations in England, maybe this is a subconscious way of developing and maintaining those positive connotations as an extension of British cultural identity in America.