We created a custom BasicAuthHttpWebRequest using the reference of the code from Solr.net and then referenced this class library in solr indexconfiguration section.

    namespace HttpWebAdapters

{

public class MyBasicAuthHttpWebRequestFactory : IHttpWebRequestFactory { private readonly string username; private readonly string password;

public MyBasicAuthHttpWebRequestFactory(string username, string password)
{//This is where we can fetch credential from appsetting
  this.username = ConfigurationManager.AppSettings["UserName"];
  this.password =ConfigurationManager.AppSettings["password"];
}

public IHttpWebRequest Create(string url)
{
  return this.Create(new Uri(url));
}

public IHttpWebRequest Create(Uri url)
{
  HttpWebRequest request = (HttpWebRequest) WebRequest.Create(url);
  string base64String = Convert.ToBase64String(Encoding.ASCII.GetBytes(this.username + ":" + this.password));
  request.Headers.Add("Authorization", "Basic " + base64String);
  return (IHttpWebRequest) new HttpWebRequestAdapter(request);
}

} }

 <indexConfigurations>
    <solrHttpWebRequestFactory type="HttpWebAdapters.HttpWebRequestFactory, SolrNet">
      <patch:delete/>
    </solrHttpWebRequestFactory>
    <solrHttpWebRequestFactory type="MyClassLibrary.MyBasicAuthHttpWebRequestFactory, MyClassLibrary">
      <param hint="username">
      </param>
      <param hint="password">
      </param>
    </solrHttpWebRequestFactory>
  </indexConfigurations>

We created a custom BasicAuthHttpWebRequest using the reference of the code from Solr.net and then referenced this class library in solr indexconfiguration section.

    namespace MyclassLibrary

{

public class MyBasicAuthHttpWebRequestFactory : IHttpWebRequestFactory { private readonly string username; private readonly string password;

public MyBasicAuthHttpWebRequestFactory(string username, string password)
{//This is where we can fetch credential from appsetting
  this.username = ConfigurationManager.AppSettings["UserName"];
  this.password =ConfigurationManager.AppSettings["password"];
}

public IHttpWebRequest Create(string url)
{
  return this.Create(new Uri(url));
}

public IHttpWebRequest Create(Uri url)
{
  HttpWebRequest request = (HttpWebRequest) WebRequest.Create(url);
  string base64String = Convert.ToBase64String(Encoding.ASCII.GetBytes(this.username + ":" + this.password));
  request.Headers.Add("Authorization", "Basic " + base64String);
  return (IHttpWebRequest) new HttpWebRequestAdapter(request);
}

} }

 <indexConfigurations>
    <solrHttpWebRequestFactory type="HttpWebAdapters.HttpWebRequestFactory, SolrNet">
      <patch:delete/>
    </solrHttpWebRequestFactory>
    <solrHttpWebRequestFactory type="MyClassLibrary.MyBasicAuthHttpWebRequestFactory, MyClassLibrary">
      <param hint="username">
      </param>
      <param hint="password">
      </param>
    </solrHttpWebRequestFactory>
  </indexConfigurations>