Return custom HTTP Header

I’m trying to return a hash of a CSV document I’m returning in a triggered task and I want to return it in a HTTP header.
I would have thought i can do it the same way i can manipulate the status code by setting a binary header, but the headers aren’t coming through.

Does anyone have an idea how to achieve this? (Ideally without buying any additional subscription services, thanks)

Here’s an example of the output of the csv formatter: