> Also, I'm not a Go developer but is this idiomatic?

The error handling is kinda strange.

I would do some like this:

    var ErrUnexpectedResponse = errors.New("unexpected response")
    ....
    func syntheticHttpRequest(url string, apiToken string) error {
        ...
        } else if resp.StatusCode != 202 {
            return ErrUnexpectedResponse
        }
        return nil
    }

(a bunch of other stuff was mentioned in the comments)