EF, Code First - How to set a custom Guid identity value on insert

I`m facing the following problem when dealing with inserting new entities in the DB that has Guid as primary keys - EF 5 with Code first approach.

I know there are a lot similar topics as I was roving for hours for this issue, but I couldn`t find topic with this problem.

As an example, my POCO class is:

public class EntityRole : IAdminModel
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public Guid Id { get; set; }

    [Required]
    [MaxLength(50)]
    public string Name { get; set; }

    [Required]
    [Display(Name = "Role code")]
    [MaxLength(20)]
    public string RoleCode { get; set; }

    [Display(Name = "Entities Assigned")]
    [InverseProperty("Role")]
    public List<Entity> Entities { get; set; }
}

RoleCode and Name are just text data, that is editable in the admin panel, so don`t consider those field names.

When adding new entity I don`t specify primary key values. So far so good, all works fine here. As a primary key field that requires value and has auto-generation of values, it is supposed not to always specify Id, but if I set a value, it should be kept (if identity insert is enabled).

But in some cases I want to specify a primary key value, for example for initial seeding of my DB with values. I need it for later processing - lets just say I need that specific Guid to be there. So if I have in my Configuration class:

// Initial data seed
protected override void Seed(WebPortalDbContext context)
{
    context.MenuItems.AddOrUpdate(
        m => m.Id,
        new EntityRole {Id = new Guid("268bf332-910e-4ea1-92f8-1ac0611f4c62"), Name = "Some name", RoleCode = "SN"},
    );
}

The Guid key setting goes not work even if I do a regular add:

using (var context = new MyDBContext())
{
    context.MenuItems.Add(
        new Entity() {Id = new Guid("<some gu