PaulS:
What is so secret about this value that you need the obscurity, and complexity, of a POST request? A GET request is far simpler, and doesn't require pissing away resources on the String class.
A GET request is supposed to be idempotent, meaning you can "get" the resource from the server multiple times without affecting the state of the server. Since the OP appears to be logging data to the server, this is not an idempotent request, so a GET request is not appropriate here, but a POST request is.
One problem I see is that you're connecting to port 443 and trying to use HTTPS, but you're not actually encrypting the request with TLS. If the server supports HTTP requests (probably on port 80), use that instead. I highly doubt you can even do TLS on the Arduino.