How can I update mongodb document for adding a new item to array?

I couldn't figure out insert to a sub array...

• _id
• MyArray
• --Item
• ----ArrayItemId
• ----Name

I want to insert items to MyArray...

How my update document should be?

MyCollection.Update( 
 new QueryDocument { { "_id", MyObject.Id } },
 new UpdateDocument { { "$set", new BsonDocument { { "MyArray", 
       new BsonArray { new BsonDocument {{ "ArrayItemId", myArrayField.Id }},
                       new BsonDocument {{ "Name", myArrayField.Name }} }}}}}, 
 UpdateFlags.None);