Can't query Pipedream from a browser with JS: failing CORS because of "redirect"

I’m trying to send a request to my Pipedream URL from my browser, using the sample code provided by Pipedream in the app, but it’s giving me this error:

Access to fetch at 'https://eo5chxnh42jg135.m.pipedream.net/' from origin 'http://127.0.0.1:5173' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: Redirect is not allowed for a preflight request.

POST https://eo5chxnh42jg135.m.pipedream.net/ net::ERR_FAILED

In Safari, this gives me an HTTP 303 error on the pre-flight check.

Any idea how to use Pipedream from JS in a browser?