I misinterpreted on my earlier reply, I have to find the reference position, not to move to the reference position which was founded earlier by homing sequence when the machine started.
In my case, it's like when the machine starts and finds the reference positions by hitting the home switches, but after one job is finished, I am changing the home switch position, so I cannot rely on the old reference position, I need to find the new reference position for one of the axis by hitting the home switch again.
If that is not possible, I would like to know if I can launch the homing sequence via gcode (to simulate CTRL+ALT+HOME shortcut).