How to asynchronously call a relation when lazy loading with Entity Framework 6.4?

I have a project that used Entity-Framework 6.4 to access data from the database.

I have the following model

public class Product
{
    public int Id { get; set; }
    public string Title { get; set; }
    public bool Available { get; set; }
    // ...
    public int CategoryId { get; set; }
    public virtual Category Category { get; set; }
}

In the controller, I want to access the Category using lazy loading. I know I can use Include() extension to eager load the relation. But instead, I want to lazy load it. Here is an example of how I would like to use it

public async Task<ActionResult> Get(ShowProduct vm)
{
    if (ModelState.IsValid)
    {
        using (var db = new DbContext())
        {
            Product model = await db.Products.FirstAsync(vm.Id);

            vm.Title = model.Title;

            if (model.Avilable)
            {
                // How can call the **Category** property using await operator?
                vm.CategoryTitle = model.Category.Title;
            }
        }
    }

    return View(vm);
}

How can I lazy load the Category relation using await operator to prevent locking the current thread?