Making an expansion for 2 mods, need help figuring out how

I'm trying to make a sort of "bridge" between Iron Furnaces and Furnace on a Stick for 1.12.2. I can't figure out how the original creator of Furnace on a Stick got the furnace GUI to open. Honestly, I don't really know what I'm doing, this mod is really for convenience on a server with friends. Any help would be appreciated.