public struct Vertex
{
[Semantic(VertexFieldSemantic.Position)]
public FVector3 position;
[Semantic(VertexFieldSemantic.Normal)]
public FVector3 normal;
[Semantic(VertexFieldSemantic.UV)]
public FVector2 texcoord;

//Specify the Alias of this field to "_BATCH_ID"
[Semantic(VertexFieldSemantic.UserData, "_BATCH_ID")]
public float batchId;

  public Vertex(FVector3 position, FVector3 normal ,FVector2 texcoord, float batchId)
  {
    this.position = position;
    this.normal = normal;
    this.texcoord = texcoord;
    this.batchId = batchId;
  }
}

private unsafe static void ExportCustomFieldToGLTF()
{
//Prepare the vertices and indices:
  var vertices = new Vertex[]
  {
    new Vertex(new FVector3(1, 0, 0), new FVector3(0, 1, 0), new FVector2(0, 0), 1),
    new Vertex(new FVector3(1, 1, 0), new FVector3(0, 1, 0), new FVector2(0, 1), 2),
    new Vertex(new FVector3(0, 1, 0), new FVector3(0, 1, 0), new FVector2(1, 0), 3),
    new Vertex(new FVector3(0, 1, 1), new FVector3(0, 1, 0), new FVector2(1, 1), 4),
  };
  var indices = new int[]
  {
    0, 1, 2,
    1, 2, 3
  };
  
  //Convert the vertices into raw bytes
  var verticesInBytes = new byte[vertices.Length * sizeof(Vertex)];
  fixed(Vertex* pSrc = vertices)
  {
    Marshal.Copy(new IntPtr(pSrc), verticesInBytes, 0, vertices.Length);
  }
  //Create a vertex declaration from reflection:
  var vd = VertexDeclaration.FromType<Vertex>();
  //construct a TriMesh from raw bytes of vertices and indices
  var mesh = TriMesh.FromRawData(vd, verticesInBytes, indices, false);
  //create a scene with the mesh
  var scene = new Scene(mesh);
  //export the scene to a binary glTF file
  scene.Save("test.glb", FileFormat.GLTF2_Binary);

  // The GLTF primitive generated in the test.glb will be:
  // {"attributes" : {"POSITION" : 0, "NORMAL" : 1, "TEXCOORD_0" : 2, "_BATCH_ID" : 3}, "mode" : 4}
}