@MarcusAureliu#s the hotkey used is the very basic :
session.attack
It's F by default since a27, for "Force Attack".
It's likely you'll want to use this when (alt or shift) sniping to avoid units to start gathering if you miss-click (in a forest for example). However it's not as good as it used to be with a26 "Ctrl" for "Attack" since now this isn't going to make an "attack move" in case you miss-click, so units will walk into suicide instead......... It would be interesting to know if someone was able to restore "Ctrl + Alt" sniping behavior from a26, which was: attack target if possible, else attack move.
@roscany A gui button is likely to be added if this is implemented. But players that don't use mouse nor keyboard are kind of an exception.... Also as previously stated, it's not going to impact you that much relative to now. The current method of sniping will still be optimal in most cases. This would be like an alternative for less spamy clicks, but less efficient, and most use cases will be to avoid baiting and for pathing in general.