To pass over a value, in the mev_ script you'd do something like this:
with(instance_create_depth(x,y,depth-1,obj_gguimenu)){
daddy = other.id
my_monster = other.menuvalue_y
//Create frames, elements and events like normal
}
(Similar to the scripts for doing stuff to party monsters, but instead of referring to the AMP we refer to a raw monster ID)