You should create an appropriate DisplayObject in advance and then pass the reference to this object in the constructor:
var myLittleCustomIcon:Sprite = new Sprite();
myLittleCustomIcon.graphics.beginFill(0x00FF00, 1);
myLittleCustomIcon.graphics.drawRect(0, 0, 100, 50);
myLittleCustomIcon.graphics.endFill();
var myCustomMaker:MyMarker = new MyMarker(myLittleCustomIcon);