Óh várj, most látom, hogy cross-origin request-et akarsz végrehajtani (kiindulva abból, hogy https:// kezdetű az URL-ed). Ebben az esetben kelleni fog ez:
An associative array of variables passed to the current script via the HTTP POST method when using application/x-www-form-urlencoded or multipart/form-data as the HTTP Content-Type in the request.
$_POST: An associative array of variables passed to the current script via the HTTP POST method when using application/x-www-form-urlencoded or multipart/form-data as the HTTP Content-Type in the request.
Alatta komment: If you want to receive application/json post data in your script you can not use $_POST. $_POST does only handle form data. Read from php://input instead.