How to send downlink to HTTP Integration from jQuery?

Anything that runs on some server will do. Even some PHP script can translate a GET into the proper POST including the secrets for the integration, and such server-side PHP script will not enforce CORS. Of course, anyone who would know the URL could invoke that, unless you come up with some security.

If you really cannot set up a server of your own (but then I wonder what value you have for your client), then something like this might help:

(I’m not sure that allows for changing the HTTP method, using something like request.request_method = 'POST'. See its examples.)