Custom app (11labs) returns blank response

I have set up a custom app to send a request to 11labs to convert text into audio.

I am receiving a code back of 200 back (which implies that the request was successful) according to their API documentation, but the message says "The application processed the request but returned a blank response. Refer to the HTTP status code above for details."

I would have instead expected an audio file like this in response:

Hey @jonbstrong

Regrettably, I must inform you that our platform no longer supports multipart/form-data, which is required for application integration. Although this content type is described in the API, it is not yet supported.