Check the Linked libcurl for HTTPS-Proxy Support
Distinguish an unsupported HTTPS proxy build from an ordinary HTTP proxy carrying HTTPS destinations.
CURLE_NOT_BUILT_IN means the requested feature is absent from the libcurl build. With an https:// proxy URL, inspect HTTPS-proxy support in the library the application actually loaded.
curl_version_info(CURLVERSION_NOW) reports that library’s version, TLS backend and feature flags. CURL_VERSION_HTTPS_PROXY identifies HTTPS-proxy capability; general HTTPS destination support is a different feature. A separate curl executable can use another library build.
First confirm the listener’s protocol. An ordinary HTTP proxy carrying an HTTPS destination still uses an http:// proxy URL. Changing that value is appropriate when it corrects the description of the listener.
If the intermediary itself requires TLS, keep its https:// URL and use a compatible library. libcurl added HTTPS proxies with OpenSSL in 7.52.0 and expanded backend support in 7.87.0. The runtime version, selected TLS backend and compiled features all matter; enabling unrelated build options does not establish this capability.