How to "Enable for Secure Social" on relationship without related item


In my data model, I have a relationship without related item.
I did that to delete automatically this object when the parent is deleted.

Now, I would like to enable Secure Social on this relationship but the button is disabled.
I'm able to do it on normal ItemTypes but not on relationship.

Is it normal?
Is there a way to enable it?

In advance, thank you for your support.