If you're using the default constructor of Poco::Net::HTTPSClientSession (or any other constructor that does not take a Poco::Net::Context::Ptr), you'll need to have an instance of Poco::Util::Application, as well as a configuration file containing the SSL/TLS configuration in order to create a default Context object and initialize the Poco::Net::SSLManager, or alternatively, initialize the default Context and SSLManager yourself.
Initializing the default Context and SSLManager involves creating a certificate handler, creating a default Context object and initializing the SSLManager. See the Mail and download samples for how this is done. Typically, the code to do this looks like this:
Poco::SharedPtr<Poco::Net::InvalidCertificateHandler> pCert =
new Poco::Net::ConsoleCertificateHandler(false);
Poco::Net::Context::Ptr pContext =
new Poco::Net::Context(
Poco::Net::Context::CLIENT_USE,
"",
"",
"rootcert.pem",
Poco::Net::Context::VERIFY_RELAXED,
9,
false,
"ALL:!ADH:!LOW:!EXP:!MD5:@STRENGTH");
SSLManager::instance().initializeClient(0, pCert, pContext);