The 6L50, 6L80, 6L90, 8L45, 8L90, 10L60, 10L80, & 10L90 transmissions from MY2015 to current are filled through a port in the bottom of the oil pan. AM-GM-51190 threads into that port and connects to the GM-45096 TransFlow. The height of the standpipe tube, attached to the transmission and extending to the bottom of the pan, sets the oil level.
Thread Size M6x1.0.