Proxy on WebBrowser in C#

using System;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace Webbrowser_Proxy
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        [DllImport("wininet.dll", SetLastError = true)]
        private static extern bool InternetSetOption(
            IntPtr dwL,
            int dw,
            IntPtr dwB,
            int dwBL
            );
        public struct SIPI
        {
            public int dwAT;
            public IntPtr pro;
            public IntPtr prB;
        }

        private void UseProxy(string Proxy)
        {
            const int PO = 38;
            const int POI = 3;

            SIPI ISI = default(SIPI);
            ISI.dwAT = POI;
            ISI.pro = Marshal.StringToHGlobalAnsi(Proxy);
            ISI.prB = Marshal.StringToHGlobalAnsi("local");

            IntPtr INS = Marshal.AllocCoTaskMem(Marshal.SizeOf(ISI));

            Marshal.StructureToPtr(ISI, INS, true);

            bool iR = InternetSetOption(IntPtr.Zero, PO, INS, Marshal.SizeOf(ISI));
        }
        private void button1_Click(object sender, EventArgs e)
        {
            webBrowser1.Navigate(textBox1.Text);
        }

        private void button2_Click(object sender, EventArgs e)
        {
            UseProxy(textBox3.Text);
            label4.Text = textBox3.Text;
        }
    }
}