I know, but you told me to discuss it at another forum
Yes, I know. But you just copied and pasted the question without answering the questions I was asking. So I didn't see any value in the new post.
That said, it looks like you're relating on the Primary key of the membership table. You can't make this column the primary key, an identity and a foreign key at the same time.
Why not do it the other way around (which is what you want anyway): e.g make aspnet_Membership.UserId the primary key and add it as a foreign key in your own table?