Mbot grid 2 custom start code

Hey my Printer need to move z axis 5 mm down before it move the other axis. So i’t trying to use custom start Gcode but it’s totaly ignore it. is it not supported yet for X3G?
Not even this works.

G21; set mm units
G28 ;home all axis
G90; set absolute coordinates
G1 X 0 Y0 Z5 F300 ;move platform down 5mm
? the build plate dossent move at all.

Help please