A injected script element was last I checked the only way to send a piece of data to a remote server on a third party domain without a preflight OPTIONS request.

Less round trips. Less bandwidth.

For more reading look here:http://dev.housetrip.com/2014/04/17/unleash-your-ajax-reques...

But in general here's what is wanted.

- No OPTIONS request

- Third party domain.

- withCredentials - pass cookies in the request.

The best option in this case is actually a injected async script element.

Source: worked in ad tech a few years ago. Looked into all the possible options, this came up least bad, and explained why it was so popular.