"Post Processing" Insert G code custom script

CHEP had a great video describing how to do multiple color prints with a single extruder using Cura slicer’s post-processing g-code. I cannot find a similar function within AstroPrint. I can alter slicer settings and review build plate and layer layout but I can’t find an option to insert g code to allow pausing a print at a certain layer. Can you point me in the right direction? He was doing this using Ultimaker Cura but I’ve always used AstroPrint so I’d like to stick with it if possible.

When the astrobox finds a M25 Gcode it will pause as of version 0.16(8)