My code is this. Please help me to integrate with your code. Thanks in advance :)

using HelixToolkit.Wpf;
using System;
using System.IO;
using System.Windows;
using System.Windows.Input;
using System.Windows.Media.Media3D;
namespace Display3DModel
{
    public partial class MainWindow : Window
    {
        private const string MODEL_PATH = "D:/vs/display3D-master/Display3DModel/bicycle.stl";
        public object FilDia { get; private set; }
        public MainWindow()
        {
            InitializeComponent();
            Model3DGroup MdlGrp = null; 

            ModelVisual3D device3D = new ModelVisual3D();
            device3D.Content = Display3d(MODEL_PATH);
            viewPort3d.Children.Add(device3D);         
        }
        private Model3D Display3d(string model)
        {
            Model3D device = null;
            try
            {
                Console.WriteLine("Hello!!!");
                viewPort3d.RotateGesture = new MouseGesture(MouseAction.LeftClick);
                ModelImporter import = new ModelImporter();
                device = import.Load(model);
            }
            catch (Exception e)
            {       
                MessageBox.Show("Exception Error : " + e.StackTrace);
            }
            return device;
        }     
    }
}

Thanks a lot :)

Here is the link of the project:

https://github.com/helix-toolkit/helix-toolkit