public class Parent {
    [DatabaseGenerated(DatabaseGeneratedOption.None)]
    public Guid Id { get; set; }
    public ICollection<Child> Children { get; set; }
}

public class Child {
    [DatabaseGenerated(DatabaseGeneratedOption.None)]
    public Guid Id { get; set; }
    public Guid ParentId { get; set; }
}

modelBuilder.Entity<Child>().HasKey(c => new { c.Id, c.ParentId });

var parent = new Parent() { Id = Guid.NewGuid() };
var child = new Child() { Id = Guid.NewGuid(), ParentId = parent.Id };
parent.Children.Add(child);

// Assume parent is already in the DB, with ID of '1b1a6ecd-00ad-4265-ac0d-9a50bd30e247'
INSERT [dbo].[Child]
   ([Id],
    [ParentId])
VALUES ('1b1a6ecd-00ad-4265-ac0d-9a50bd30e247' /* @0 */,
    '1b1a6ecd-00ad-4265-ac0d-9a50bd30e247' /* @1 */)