How can I construct an HTTPS POST with drupal_http_request?
You should only need to set it in the protocol.

$result = drupal_http_request('https://somewhere.com', $options);


Parameters


$url:
A string containing a fully qualified URI.

array $options:
(optional) An array that can have one or more of the following elements:


headers: An array containing request headers to send as name/value pairs.
method: A string containing the request method. Defaults to &#039;GET&#039;.
data: A string containing the request body, formatted as
&#039;param=value&amp;param=value&amp;...&#039;. Defaults to NULL.
max_redirects: An integer representing how many times a redirect
may be followed. Defaults to 3.
timeout: A float representing the maximum number of seconds the function
call may take. The default is 30 seconds. If a timeout occurs, the error
code is set to the HTTP_REQUEST_TIMEOUT constant.
context: A context resource created with stream_context_create().