Hi Chad,
to check the payload of the request open the browser developer tools (F12), go to the network tab, do the action which triggers the post request and then check the payload.
On the other side in the XSJS in the debugger mode the body has to be the same than the payload.
As you can see I have implemented your little example and I works w/o issues. If have tried it from an UI5 app running on XS and an UI5 app running on HCP consuming the XSJS service via a destination.
Best Regards,
Florian