I have the exact same issue - Cura is inserting both M140 and M104 commands prior to the custom g-code that I have setup, which is to heat the nozzle to 160 and bed to 60 to perform a auto-home followed by a bilinear ABL. This is then followed by heating the nozzle to the layer 0 temp and bed to layer 0 bed temp. How do I get rid of the initial M140 and M104 commands that Cura is inserting?