How to set existing DB with ASP.NET identity?

I want to set my existing database TestDB into my application so that Identity will create their tables inside this database.

But I am getting error:

Cannot attach the file 'D:\TestApp\TestApp\App_Data\TestDB.mdf' as database 'TestDB'.

I have done couple of things which are

1. I removed defaultconnection to TestDBConnection in web.config

   <add name="TestDBConnection"
   connectionString="Data Source=(LocalDb)\MSSQLLocalDB;AttachDbFilename=|DataDirectory|\TestDB.mdf;Initial Catalog=TestDB;Integrated Security=True"
   providerName="System.Data.SqlClient" />
   

2. I changed in IdentityModels.cs

   public class ApplicationDbContext : IdentityDbContext<ApplicationUser>
   {
      public ApplicationDbContext()
           : base("TestDBConnection", throwIfV1Schema: false)
      { }
   

3. In package Manager Console,

   PM>enable-migrations
   

4. In Configuration.cs,

   internal sealed class Configuration : 
   DbMigrationsConfiguration<TestApp.Models.ApplicationDbContext>
   {
       public Configuration()
       {
           AutomaticMigrationsEnabled = true;
       }
   

Please guide me where am I going wrong. I am looking for your suggestions.