The intersect field will take data for a runway intersection takeoff - either the displacement in meters, or the name of the intersection.
In the real plane the position of the FMS will update to the runway beginning when you press the TOGA buttons. If you take off from an intersection, this update would be erroneous, since you are not actually at the beginning. So you can enter an intersection value (i.e. 900m) and the updated position will be correct.
If the FMS position is being fed GPS data, this update will not be taken into account - so it does not matter if you enter anything or not. The IXEG 737 will always have GPS updating, so entering anything (or nothing) into this field will have no effect.
Cheers, Jan