How to export isosurface

I using Netgen to get an isosurface with parameters and get a contour of mesh fig-a. Now, with the contour I can get an Isosurface, fig-b, I wish to export the isosurface, as a neutral file, how to do it?

The codes are below:
import ngsolve as ng
import netgen.occ as occ
import numpy as np
import netgen.gui
from netgen.geom2d import SplineGeometry
from netgen.csg import unit_cube
from netgen.csg import CSGeometry
from netgen.csg import Pnt
from netgen.csg import OrthoBrick
from ngsolve import *
cube = OrthoBrick( Pnt(0,0,0), Pnt(1,1,1) ).bc(“Gamma”)
geo = CSGeometry()
geo.Add (cube)
Om_mesh = Mesh(geo.GenerateMesh(maxh=0.1))
Draw(myfunc,Om_mesh, “derivative”)