how to add sub collection to a document in firestore? (Updated for for web v 9)

Question

I have a similar question to how to add sub collection to a document in firestore?, but I am using web version 9. The firebase documentation reads values in a subcollection, but I am trying to add a document to a subcollection.

I just need an example on how to add a document using web version 9 because I can not find one.