C#: Create Index For An Attribute In Embedded Document

I have following document in my MongoDB database:

{
   "_id":1,
   "user_name":"John Doe",
   "addresses":[
      {
         "_id":null,
         "geolocation":null,
         "city":"Toronto"
      },
      {
         "_id":null,
         "geolocation":null,
         "city":"Canada"
      }
   ]
}

I want to create an index for the attribute addresses.city in my C# code so that I can do a text based search later. How can I achieve this?

By the way, I have implemented the following code to achieve this but it seems it doesn't work while running the query:

public void createIndexes() {
  try {
    var indexOptions = new CreateIndexOptions();
    var indexKeys = Builders < Users> .IndexKeys.Text(_ => _.Addresses);
    var indexModel = new CreateIndexModel < Users> (indexKeys, indexOptions);

    var collection = _mongoDb.GetCollection < Users> ("users");
    collection.Indexes.CreateOneAsync(indexModel);
  } catch (Exception ex) {
    throw;
  }
}

The code executing the query:

//Creates connection with the mongodb
private MongoDbContext db = new MongoDbContext();
...
...

//Initializes the search term
string searchTerm = "Cana";

//Executes the query
var builder = Builders<Users>.Filter;
FilterDefinition<Users> filter;
filter = builder.Text(searchTerm);
var results = await db.Users.Find(filter).ToListAsync();

I'm using this MongoDB driver