Non-array output

I think you might just be confused by their documentation? That is what a POST looks like in raw form, at least x-www-form-urlencoded, which I think is what WordPress uses, as opposed to form-data…

Failing that, you could modify how Xpost processes and add an extra format “url” like ‘json’ at lines:
* https://github.com/zaus/forms-3rdparty-xpost/blob/master/forms-3rdparty-xpost.php#L86
* https://github.com/zaus/forms-3rdparty-xpost/blob/master/forms-3rdparty-xpost.php#L194

which can just set $args['body']=http_build_query($args['body']), like the JSON handling does.

You could just write the hook directly — use add_filter('Forms3rdPartyIntegration_service_filter_args', 'myurl_post_args_fn', 12, 3); and make the above change.