Aether Mortar?

[ICountFrom0]

MCH's ability set.

"Aether Mortar" is listed as a possible ability, but there dosen't seem to be any way to actually use it.

(and yes, I've found the buff from placing a bishop autoturret and standing in it)
(and YES, I've found that if you click the aether mortar in the pvp menue it'll trigger, but ... "cannot be placed on bar")

.... so, how do you get it on the bar?