Easy. Write "emergency medicine (EM)" the first time, then call it EM the rest of the way. Ditto for "emergency department (ED)".
As for ER, only the bungholes on this board get all bent out of shape about this term. At my northeastern program, and at southern programs where I have friends, and at my cousin's Cali residency, people routinely - and I mean ROUTINELY - refer to themselves as ER docs, and to the specialty itself as ER.
Yeah, you can argue that surgeons don't call themselves "OR docs" but nothing is consistent or logical in the English language, least of all recent medical parlance.
It's best on the app to guard yourself against fanatics by using the term EM, but realize that in the real world, many of us rational people call our specialty ER.
Opening salvo in the "Never call it ER" debate....