Adding Array to Firestore document

Hello, I am trying to automate acuity scheduling by sending the data coming from appointments to Firestore. My scenario looks like this:

What I am trying to achieve is to transform the data that come like this from Acuity:

In Firestore documents that look like this:

Obviously my approach is not working, and I read that I might need the Array Aggregator but I do not understand it very well and can’t seem to make the connection properly to get the desired outcome.