How do I create CSV that the API can parse from a string?

I am looking to create a CSV from a string that I can send to a Qualtrics API to run a call. The call I am trying to run is at this URL:
https://api.qualtrics.com/reference#start-response-import
The call asks for a CSV file, but I don't know how to give it a CSV; I only know how to download that CSV using
var CSVAsString = '1,2,3'
var CSV = 'data:attachment/csv,' + encodeURIComponent(CSVAsString)
How would I send a CSV instead of a string of text that can be used to download the CSV? (I don't have trouble sending it, just have trouble creating a CSV document that the APIs can read)