What should I do to automatically link

I need a help.

Collection “A” has an “id” column, and collection “B” has an “Aid” column.
What should I do to automatically link “Aid” to the “id” column with “Many to one”?

I asked on perplexity and got the answer:
Specify “$parent.A.id” in the “Default values” section.
But it seems there is no such thing as a “Default values” section.

What should I do?
Thanks in advance.

Create a “many to one” field in B