CURLOPT_SSLKEY (3) - Linux Manuals
CURLOPT_SSLKEY: specify private keyfile for TLS and SSL client cert
NAMECURLOPT_SSLKEY - specify private keyfile for TLS and SSL client cert
DESCRIPTIONPass 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.