Delaying your turn is not allowed in 5th edition, though it was in previous editions.
What you are thinking about is taking the Ready Action, which is to say you decide what to do and how it will trigger (such as I ready an action to attack the goblin as soon as an ally is within 5' of them). Then, when conditions are met, you may use your reaction to trigger this readied action.
Since it's EA and such they were never going to manage to implement this right now - so I suspect it'll be 3 months minimum before they can decide how to do this (because RAW is not possible in a video game, so they will have to alter the rules to implement it).