CURLOPT_SSLKEY man page
CURLOPT_SSLKEY — specify private keyfile for TLS and SSL client cert
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSLKEY, char *keyfile);
Pass a pointer to a zero terminated string as parameter. The string should be the file name of your private key. The default format is "PEM" and can be changed with CURLOPT_SSLKEYTYPE(3).
(iOS and Mac OS X only) This option is ignored if curl was built against Secure Transport. Secure Transport expects the private key to be already present in the keychain or PKCS#12 file containing the certificate.
The application does not have to keep the string around after setting this option.
All TLS based protocols: HTTPS, FTPS, IMAPS, POP3S, SMTPS etc.
If built TLS enabled.
Returns CURLE_OK if TLS is supported, CURLE_UNKNOWN_OPTION if not, or CURLE_OUT_OF_MEMORY if there was insufficient heap space.
curl_easy_setopt(3), CURLOPT_KEYPASSWD(3), CURLOPT_PROXY_KEYPASSWD(3), CURLOPT_SSLCERT(3), CURLOPT_SSLCERTTYPE(3), CURLOPT_SSLENGINE(3), CURLOPT_SSLKEYTYPE(3).