CURLOPT_KEYPASSWD man page
CURLOPT_KEYPASSWD — set passphrase to private key
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_KEYPASSWD, char *pwd);
Pass a pointer to a zero terminated string as parameter. It will be used as the password required to use the CURLOPT_SSLKEY(3) or CURLOPT_SSH_PRIVATE_KEYFILE(3) private key. You never needed a pass phrase to load a certificate but you need one to load your private key.
The application does not have to keep the string around after setting this option.
All TLS based protocols: HTTPS, FTPS, IMAPS, POP3S, SMTPS etc.
This option was known as CURLOPT_SSLKEYPASSWD up to 7.16.4 and CURLOPT_SSLCERTPASSWD up to 7.9.2.
Returns CURLE_OK if TLS enabled, CURLE_UNKNOWN_OPTION if not, or CURLE_OUT_OF_MEMORY if there was insufficient heap space.