How to update the value of a document that was just inserted in a database? (Beginner)

Hey everyone, I'm learning how to use Appwrite and so far it's been going really well!
I have a question though, say we have a function that is executed whenever a document is added to a collection.

If I wanted to update the value of one of that document's fields based on another of its fields, what would the best course of action be?
I tried checking the "req" object's body and performing my conditional logic and it works like a charm, however when attempting to update my new value via databases.updateDocument it seems to do nothing.

I'm pasting my code below...

https://pastebin.com/um5VhfUX

Thank you a lot!

You don’t seem to have authenticated to Appwrite anywhere here

You need to either use client.setKey() to use server side functions with an API key, or client.setJWT() to authenticate as a user (with a JWT you created on the client side).