How do I add a record to a specific collection when adding a new record to the selected collection?

In this scenario, the user has created a record (payments) and is adding it to a collection (categories). When the user selects the category, it adds the created payment record to that record but I also want to make a payment that is added to a specific category called “Unspent.”

I can target items in a list like :first-thing or :last-thing but can I update a specific item?

Yes you can… if you know what item it is in the list you can refer to that (i.e. list: item#14)

But you don’t need to do that… you can just define the thing in question directly (most probably with a search).