just download ptr client, go into keybinds (which are almost totally reworked) and you'll find it in section targeting. i thought it was obvius how to loot on slaves. just assist the main while he is targetting the dead mob and hit interact with target button. another cool thing is if you have "click-to-move" on your toon will run to loot if they are not in range.
i added screenshot from ptr. i think there are so much more opportunities ist those two keybindings looting is just one of them.