Insert entity with many to many related entities which exist in the database

I have two objects with many to many relationship:

public class Order
{
   public int OrderID { get; set; }
   public string OrderName { get; set; }
   public virtual Collection<Product> Products { get; set; } = new Collection<Product>();
}

public class Product
{
   public int ProductID { get; set; }
   public string ProductName { get; set; }
   public int OrderID { get; set; }
   public virtual Collection<Order> Orders { get; set; } = new Collection<Order>();
}

// Mapping table
public class OrderProduct
{
   public int OrderId { get; set; }
   public Order Order { get; set; }
   public int ProductId { get; set; }
   public Product Product { get; set; }
}

I would like to add a new Order with a collection of existing Products (my API would have an input of ProductID array) into the database, so I perform like:

private void AddOrder(int[] productIDs)
{
    Order newOrder = new Order();
    newOrder.Name = "New Order";

    // Here I have no clue which would be the correct way...Should I do 
    // Approach A(fetch each related entity - Product from database then add them into the collection of my new base entity - Order):
    productIDs.ToList().Foreach(p => 
        {
            newOrder.Products.Add(_dbContext.Product.FindById(p))
        }
    );

    _dbContext.Orders.Add(newOrder);
    var newOrderID = _dbContext.SaveChanges();

    // then fetch each product from database and add my new Order to its collection and save
    productIDs.ToList().Foreach(p => 
        {
            var existedProductFromDb = _dbContext.Product.FindById(p)ï¼›
            existedProductFromDb.Orders.Add(newOrder);
            _dbContext.SaveChanges();
        }
    );
}

Do I really need the Mapping table between Order and Product in Entity Framework Core? Otherwise, what would be the correct way to deal with above scenario?