I am the eldest and still unmarried. My younger brother wanted to enter shidduchim and I agreed.
The Shadchonit was also married to the Av Beis Din. She sought the permission of her husband. Both of us sat before him (Av Beis Din) and he formally asked me if I gave permission to my brother to marry before me.
I did so and then he told the Shadchonit to go ahead and make suggestions to my borther.
It that way my permission had been concretized.