IE - C#, How to inject a JavaScript file to a web page?

I am working on IE extension development. I would like to inject an external JS file into the browser(IE) when it opens.

Please refer this code example may help to solve your issue.

(1) Add references and imports to the following components:

using SHDocVw;
using mshtml;
using System.IO;
using Microsoft.Win32;
using System.Runtime.InteropServices; 

(2) Define the IOleObjectWithSite interface just above your BHO class declaration like this:

[
    ComVisible(true),
    InterfaceType(ComInterfaceType.InterfaceIsIUnknown),
    Guid("FC4801A3-2BA9-11CF-A229-00AA003D7352")
]
public interface IObjectWithSite
{
    [PreserveSig]
    int SetSite([MarshalAs(UnmanagedType.IUnknown)]object site);
    [PreserveSig]
    int GetSite(ref Guid guid, out IntPtr ppvSite);
}

(3) Make the BHO class implement the IOleObjectWithSite interface

[
        ComVisible(true),
        Guid("2159CB25-EF9A-54C1-B43C-E30D1A4A8277"),
        ClassInterface(ClassInterfaceType.None)
]

public class BHO : IObjectWithSite
{
  private WebBrowser webBrowser;

  public int SetSite(object site)
  {
    if (site != null)
    {
        webBrowser = (WebBrowser)site;
        webBrowser.DocumentComplete += 
          new DWebBrowserEvents2_DocumentCompleteEventHandler(
          this.OnDocumentComplete);
    }
    else
    {
        webBrowser.DocumentComplete -= 
          new DWebBrowserEvents2_DocumentCompleteEventHandler(
          this.OnDocumentComplete);
        webBrowser = null;
    }

    return 0;

  }

  public int GetSite(ref Guid guid, out IntPtr ppvSite)
  {
    IntPtr punk = Marshal.GetIUnknownForObject(webBrowser);
    int hr = Marshal.QueryInterface(punk, ref guid, out ppvSite);
    Marshal.Release(punk);
    return hr;
  }

  public void OnDocumentComplete(object pDisp, ref object URL)
  {
    HTMLDocument document = (HTMLDocument)webBrowser.Document;
  }

}

(4) Implement the OnDocumentComplete method to inject the JavaScript code and the div element

public void OnDocumentComplete(object pDisp, ref object URL)
{
    HTMLDocument document = (HTMLDocument)webBrowser.Document;

    IHTMLElement head = (IHTMLElement)((IHTMLElementCollection)
                           document.all.tags("head")).item(null, 0);
    IHTMLScriptElement scriptObject = 
      (IHTMLScriptElement)document.createElement("script");
    scriptObject.type = @"text/javascript";
    scriptObject.text = "\nfunction hidediv(){document.getElementById" + 
                        "('myOwnUniqueId12345').style.visibility = 'hidden';}\n\n";
    ((HTMLHeadElement)head).appendChild((IHTMLDOMNode)scriptObject);

    string div = "<div id=\"myOwnUniqueId12345\" style=\"position:" + 
                 "fixed;bottom:0px;right:0px;z-index:9999;width=300px;" + 
                 "height=150px;\"> <div style=\"position:relative;" + 
                 "float:right;font-size:9px;\"><a " + 
                 "href=\"javascript:hidediv();\">close</a></div>" +
        "My content goes here ...</div>";

    document.body.insertAdjacentHTML("afterBegin", div);
}

(5) Register your BHO to be loaded by Internet Explorer

public const string BHO_REGISTRY_KEY_NAME = 
   "Software\\Microsoft\\Windows\\" + 
   "CurrentVersion\\Explorer\\Browser Helper Objects";

[ComRegisterFunction]
public static void RegisterBHO(Type type)
{
    RegistryKey registryKey = 
      Registry.LocalMachine.OpenSubKey(BHO_REGISTRY_KEY_NAME, true);

    if (registryKey == null)
        registryKey = Registry.LocalMachine.CreateSubKey(
                                BHO_REGISTRY_KEY_NAME);

    string guid = type.GUID.ToString("B");
    RegistryKey ourKey = registryKey.OpenSubKey(guid);

    if (ourKey == null)
    {
        ourKey = registryKey.CreateSubKey(guid);
    }

    ourKey.SetValue("NoExplorer", 1, RegistryValueKind.DWord);

    registryKey.Close();
    ourKey.Close();
}

[ComUnregisterFunction]
public static void UnregisterBHO(Type type)
{
    RegistryKey registryKey = 
      Registry.LocalMachine.OpenSubKey(BHO_REGISTRY_KEY_NAME, true);
    string guid = type.GUID.ToString("B");

    if (registryKey != null)
        registryKey.DeleteSubKey(guid, false);
}

For more detailed information and instructions, you can refer link below.

Inject HTML and JavaScript into an existing page with BHO using MS Visual Studio 2010 and C#