You need create a new OccluderPolygon2D resource and in this new resource you pass the PoolVector2Array with the points
var light_occluder = LightOccluder2D.new()
add_child(light_occluder)
var occluder_polygon = OccluderPolygon2D.new()
occluder_polygon.polygon = PoolVector2Array([Vector2(0,0), Vector2(0,70), Vector2(70,0), Vector2(70,70)])
light_occluder.set_occluder_polygon(occluder_polygon)